Porous one-dimensional polymeric graphitic carbon nitride-based nanosystems for catalytic conversion of carbon monoxide and carbon dioxide under ambient conditions

ABSTRACT

In some aspects and embodiments, the present application provides a wide range of porous 1-D polymeric graphitic carbon-nitride materials that are atomically doped with binary metals in different morphologies. In some embodiments, the graphitic carbon-nitride materials can be prepared with high mass production from inexpensive and natural abundant precursors. In some embodiments, the materials were used successfully for the oxidation of CO to CO2 under ambient reaction temperature in addition to the reduction of CO2 into hydrocarbons. In some embodiments, the materials can be used for practical and large-scale gas conversion for household or industrial applications.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of US Provisional Application No. 62/960,946 (filed Jan. 14, 2020), which is incorporated by reference in its entirety for all purposes.

TECHNICAL FIELD

The present application sets forth porous one-dimensional polymeric graphitic carbon nitride-based nanosystems and their applications in catalytic conversion of carbon monoxide and carbon dioxide.

BACKGROUND

Global energy consumption has substantially increased in the last decades, and it is expected to rise by more than 40% in the coming 20 years, owing to continued economic and population growth (FIG. 1A). This has led to massive emissions of CO and CO₂ gases, which will rise around 15% in the coming 10 years (FIG. 1B). Over 35 billion metric tons of CO and CO₂ gasses are emitted annually worldwide from human sources. For example, Qatar Aluminium company in Qatar produces around 4,987,233 mt of CO_(2eq) (nearly 7.63 tons of CO_(2eq) per ton of aluminum) and is expected to increase by around 20% in the coming 10 years. Meanwhile, around 1,319,563 cars in Qatar emit nearly 98,967.225 tons of CO annually.

Given the increased use of energy and the generation of CO and CO₂ gasses, the successful conversion of CO and CO₂ into useful hydrocarbon fuels could be a sustainable energy source worth billions of dollars. The design and fabrication of efficient and inexpensive catalysts that are applicable to commercial usage as well as development of a mobile gas conversion system could provide great social and economic benefits.

BRIEF SUMMARY

In some aspects, provided herein is a graphitic-like carbon nitride nanostructure, which is optionally doped atomically with one or more metal elements. In some embodiments, the carbon nitride nanostructure can be nanotube, nanowire, nanorod, or nanofiber. In some embodiments, the carbon nitride nanostructure comprises nanontubes, nanowires, nanorod, or nanofibers.

In some aspects, provided herein is a method of making a metal-doped carbon nitride nanostructure, comprising:

providing a metal salt solution comprising one or more metal salts and a first solvent;

adding melamine to the metal salt solution followed by adding an acid solution, optionally while stirring at a first temperature, thereby forming a precipitate;

washing the precipitate with a second solvent; and

drying the washed precipitate at a second temperature, thereby obtaining a powder.

In some aspects, provided herein is a carbon nitride nanostructure made by the method of any embodiments provided herein.

In some aspects, provided herein is a catalyst comprising the carbon nitride nanostructure of any embodiments provided herein.

In some aspects, provided herein is a method of oxidizing CO, comprising: providing a CO gas mixture comprising at least 1% CO; providing a catalyst described herein to the CO gas mixture; and performing a CO oxidation reaction.

In some aspects, provided herein is a method of reducing CO₂, comprising: providing a CO₂ gas mixture containing at least 1% CO; providing a catalyst described herein to the CO₂ gas mixture; and performing CO₂ reduction reaction.

In some aspects, provided herein is a method of converting CO or CO₂ to a hydrocarbon, comprising: providing a CO or CO₂ gas mixture containing 1-100% CO or CO₂ respectively; providing a catalyst of any of any embodiments provided herein to the gas mixture; and performing conversion reaction.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A-1B show the worldwide energy consumption (FIG. 1A) and worldwide CO₂ emission (FIG. 1B), as obtained from International Energy Outlook 2019, U.S. Energy Information Administration Office of Energy Analysis, U.S. Department of Energy, Washington, D.C. 20585.

FIGS. 2-10B are directed to Example 1 described herein.

FIG. 2 shows a scheme (Scheme 1) displaying an exemplary fabrication process of a metal-doped carbon nitride nanostructure Au/Pd/gC₃N₄NTs, the process comprising or consisting of the polymerization of melamine in an ethylene glycol solution in the presence of both Pd and Au precursors with the assistance of nitric acid followed by carbonization.

FIGS. 3A-3D show microscopy of Au/Pd/gC₃N₄NTs at different magnification scales, including scanning electron microscopy (SEM) images at 1 mm (FIG. 3A) and 200 nm (FIG. 3B) and transmission electron microscopy (TEM) images at 500 nm (FIG. 3C) and 200 nm (FIG. 3D).

FIGS. 4A-4C show a high-magnification TEM image of Au/Pd/gC₃₎N₄NTs (FIG. 4A) and HRTEM images of the numbered areas in FIG. 4A respectively (FIGS. 4B and 4C). The inset in FIG. 4C shows the FFT and FT.

FIGS. 5A-5E show a HAADF-STEM image of a single Au/Pd/gC₃N₄NT nanotube (FIG. 5A) and its elemental mapping analysis for C (FIG. 5B), N (FIG. 5C), Au (FIG. 5D), and Pd (FIG. 5E). FIG. 5F shows an EDX analysis of the as-fabricated materials. The indicated scale bars in FIGS. 5B-5D are 20 nm.

FIGS. 6A-6F show wide-angle XRD patterns (FIG. 6A) and XPS survey (FIG. 6B) of Au/Pd/gC₃N₄NTs and gC₃N₄ NTs, as well as high-resolution XPS spectra of C 1s (FIG. 6C), N 1s (FIG. 6D), Au 4f (FIG. 6E), and Pd 3d (FIG. 6F).

FIGS. 7A-7D show N₂ adsorption-desorption isotherms and pore size distributions of Au/Pd/gC₃N₄NTs (FIGS. 7A and 7B) and gC₃N₄NTs (FIGS. 7C and 7D), respectively.

FIG. 8 shows FTIR analysis of Au/Pd/gC₃N₄NTs as compared to gC₃N₄NTs.

FIGS. 9A-9D show the CO conversion efficiency of several exemplary catalysts as a function of temperature (FIG. 9A), the CO conversion kinetics at temperatures that ranged from 25 to 150° C. (FIG. 9B), the CO conversion as a function of time (FIG. 9C), and the stability tests of An/Pd/gC₃N₄NTs measured for 10 cycles (FIG. 9D).

FIGS. 10A-10B show CO peaks adsorbed on Au/Pd/gC₃N₄NTs and gC₃N₄NTs benchmarked in a CO-saturated aqueous solution of 0.5 M KaOH at a sweeping rate of 50 mV (FIG. 10A) and CO-oxidation kinetics at a potential ranging between −0.8 and 0.5 V (FIG. 10B).

FIGS. 11-19B are directed to Example 2 described herein.

FIG. 11 shows a scheme (Scheme 2) displaying an exemplary preparation process of Au/Pd/gC₃N₄NFs.

FIGS. 12A-12B show SEM (FIG. 12A) and TEM (FIG. 12B) images of Au/Pd/gC₃N₄NFs.

FIGS. 13A-13C show a high magnification TEM image of Au/Pd/gC₃N₄NFs (FIG. 13A) and HRTE (FIGS. 13B and 13C).

FIGS. 14A-14F show a HAADF-STEM image of a single Au/Pd/gC₃N₄NFs nanofiber (FIG. 14A) and elemental mapping images for C (FIG. 14B), N (FIG. 14C), Au (FIG. 14D), and Pd (FIG. 14E), respectively, as well as EDX (FIG. 14F).

FIGS. 15A-15F show wide-angle XRD patterns (FIG. 15A) and XPS survey (FIG. 15B) of Au/Pd/gC₃N₄NFs and gC₃N₄NFs, as well as high-resolution XPS spectra of C (FIG. 15C), N (FIG. 15D), Au (FIG. 15E), and Pd (FIG. 15F).

FIGS. 16A-16B show N₂-physisorption isotherms of Au/Pd/gC₃N₄NFs (FIG. 16A) and gC₃N₄NFs (FIG. 16B).

FIGS. 17A-17C show TEM images of Au/Pd/gC₃N₄ nanostructures prepared by fast addition of melamine (FIG. 17A), 60 ml of HNO₃ (0.05 M) (FIG. 17B), and quick addition of nitric acid (FIG. 17C).

FIGS. 18A-18D show the CO conversion efficiency on some exemplary catalysts as a function of temperature (FIG. 18A), the CO conversion efficiency on different catalysts at a temperature ranging between 25 and 200° C. (FIG. 18B), the total conversion temperature T₁₀₀ (FIG. 18C), and the durability tests for CO conversion percentage as a function of time (FIG. 18D).

FIGS. 19A-19B show a SEM image of Au/Pd/C₃N₄NFs before (FIG. 19A) and after (FIG. 19B) durability tests.

FIGS. 20-29 are directed to Example 3 described herein.

FIG. 20 shows a scheme (Scheme 3) displaying the formation process of Pt/Pd/CNs nanorods.

FIGS. 21A-21D show the SEM (FIGS. 21A-21B), TEM (FIG. 21C), and SAED pattern (FIG. 21D) of Pt/Pd/CNs nanorods.

FIGS. 22A-22C show a high magnification TEM image of Pt/Pd/CNs (FIG. 22A) and HRTEM images of the marked area in the shell (FIG. 22B) and in the core (FIG. 22C); the insets in FIGS. 22B-22C show the FFT image.

FIGS. 23A-23E show a HAADF-STEM image of Pt/Pd/CNs nanorods (FIG. 23A) and its elemental mapping for C (FIG. 23B), N (FIG. 23C), Pt (FIG. 23D), and Pd (FIG. 23E).

FIGS. 24A-24F show an XRD spectra (FIG. 24A) and XPS survey (FIG. 24B) of Pt/Pd/CNs nanorods and gCN nanorods, as well as high-resolution XPS spectra of C 1 s (FIG. 24C), N is (FIG. 24D), Pt 4f (FIG. 24E), and Pd 3d (FIG. 24F).

FIGS. 25A-25B show N₂ adsorption-desorption isotherms of Pt/Pd/CNs nanorods (FIG. 25A) and metal-free CN nanorods (FIG. 25B).

FIGS. 26A-26D show CVs (FIG. 26A) and LSV (FIG. 26B) of Pt/Pd/CNs, CNs, and Pt/C benchmarked in a CO-saturated aqueous solution of 0.1 M KOH at a sweeping rate of 50 mVs⁻¹; comparison of the current densities and oxidation potentials (FIG. 26C); and ECSA of the as-synthesized catalysts (FIG. 26D).

FIGS. 27A-27F show CVs measured at different scan rates in a CO-saturated aqueous solution of 0.1 M KOH at 50 mVs⁻¹ (FIGS. 27A, 27C, and 27E) and the corresponding plots of forward peak currents against the square root of the scan rates (FIGS. 27B, 27D, and 27F) of Pt/Pd/CNs nanorods (FIGS. 27A-27B), commercial Pt/C catalyst (FIGS. 27C-27D), and CN nanorods (FIGS. 27E-27F).

FIGS. 28A-28D show I-T chronoamperometric measurements in a CO-saturated aqueous solution of 0.1 M KOH at −0.19 V (FIG. 28A), as well as CVs before and after durability cycles for Pt/Pd/CNs nanorods (FIG. 28B), CN nanorods (FIG. 28C), and commercial Pt/C catalyst (FIG. 28D).

FIG. 29 shows CVs of Pt/Pd/CNs nanorods measured under UV-light irradiation relative to that under dark in a CO-saturated aqueous solution of 0.1 M KOH at 50 mV s⁻¹.

FIGS. 30-36E are directed to Example 4 described herein.

FIG. 30 shows oxidation of CO to CO₂ and reduction of CO₂ to HCOOH using the catalyst Pd/Cu/gC₃N₄NTs.

FIGS. 31A-31F show SEM (FIG. 31A) and TEM (FIG. 31B) images of Pd/Cu/gC₃N₄NTs, as well as a high magnification TEM image of an individual nanotube (FIG. 31C), HRTEM images of the numbered areas in FIG. 31C, respectively (FIGS. 31D-31E), and the SAED pattern of Pd/Cu/gC₃N₄NTs (FIG. 31F).

FIGS. 32A-32F show a HAADF-STEM image of a single Pd/Cu/gC₃N₄NTs nanotube (FIG. 32A) and its elemental mapping analysis for C (FIG. 32B), N (FIG. 32C), Pd (FIG. 32D), and Cu (FIG. 32E), respectively, and EDX analysis of gC₃N₄NTs and Pd/Cu/gC₃N₄NTs (FIG. 32F).

FIGS. 33A-33F show a wide-angle XRD pattern (FIG. 33A), an XPS survey of Pd/Cu/gC₃N₄NTs and gC₃N₄NTs (FIG. 33B), and high-resolution XPS spectra of C 1s (FIG. 33C), N 1s (FIG. 33D), Pd 3d (FIG. 33E), and Cu 2p (FIG. 33F).

FIGS. 34A-34D show N₂ adsorption-desorption isotherms and pore size distributions of Pd/Cu/gC₃N₄NTs (FIG. 34A and FIG. 34C) and gC₃N₄NTs (FIG. 34B and FIG. 34D), respectively.

FIGS. 35A-35B show the CO conversion efficiency of some exemplary catalysts as a function of temperature (FIG. 35A) and CO conversion kinetics at temperatures that ranged from 25 to 200° C. (FIG. 35B).

FIGS. 36A-36E show CVs in the absence of CO₂ (FIG. 36A), CVs (FIG. 36B), LSV (FIG. 36C), and EIS in the presence of CO₂ (FIG. 36D) measure in 0.5 M NaHCO₃ under an applied potential of 0.2 V at room temperature, as well as CVs under light relative to under dark on a Pd/Cu/gC₃N₄CTs tested CO₂-saturated aqueous solution of 0.5 M NaHCO₃ at 50 nVs⁻¹ at room temperature (FIG. 36E).

DETAILED DESCRIPTION

In some aspects, provided herein is a solution for conversion of CO₂ and CO gases into useful hydrocarbon fuels under ambient reaction conditions using active, stable, inexpensive, earth-abundant, and environmental benign carbon-nitride-based catalysts to reduce greenhouse-gas emissions.

In some aspects, provided herein are porous one-dimensional (1-D) polymeric graphitic-like carbon nitrides (gC₃N₄) nanotubes, nanowires, nanorods, and nanofibers codoped atomically with Au/Pd or Pd/Cu or Pt/Pd as active and durable gas phase catalysts and electrocatalysts for CO and CO₂ conversion reactions. In some aspects and embodiments, the catalysts provided herein possess various physicochemical benefits, including unique thermal stability (up to 600° C. in air), dynamic chemical stability in different organic/inorganic solvents, high electron density, massive defects, and nontoxicity which are highly required merits in practical gas conversion applications.

In some aspects and embodiments, porous 1-D gC₃N₄ nanostructures enriched with N-atom provide various accessible active sites for the adsorption of the reactant molecules and facilitate their diffusion to the interior cores, which are not vulnerable to aggregation and have high tolerance to the reaction products. Meanwhile, the abundant dipole N⁻—C⁺ bonds inside gC₃N₄ favor electrophilic/nucleophilic attack, accelerating the adsorption of CO along with the adsorption/dissociation of O₂, which are pivotal factors for enhancement the gas conversion performance.

Unlike traditional supported metal and metal oxides catalysts ubiquitously used with a high content (nearly ≥30 wt. %), some embodiments of the catalysts provided herein are freestanding and include Au/Pd, Pd/Cu, and Pt/Pd in the form of atomic doping with content thereof (e.g., only 1 wt. %-1.2 wt. %). In some preferred embodiments, the atomic doping reduces consumption of expensive and scarce metals and provides more accessible active sites for the adsorption of reactant molecules, generating oxygenated species and accelerating the gas conversion kinetics under ambient conditions with a high tolerance for the reaction products. In some embodiments, the doping increases the utilization of metal atoms and reduces or eliminates susceptibility for agglomeration during the gas conversion reactions.

In some embodiments, the 1-D gC₃N₄-based materials provided herein can be easily prepared from wide ranges of earth abundant and inexpensive resources, which can be adapted to large-scale and practical applications.

In some embodiments, the combination between unique physicochemical merits of 1-D gC₃N₄ and impressive catalytic properties of Au/Pd, Pd/Cu, and Pt/Pd resulted in efficient catalysts in line with the United States Department of Energy target for CO oxidation catalyst (<150° C.).

In some aspects and embodiments, the catalysts provided herein can fully oxidize CO to CO₂ and reduce CO₂ to hydrocarbon thermally, electrochemically, or photo-electrochemically.

In some embodiments, the catalytic performances of the catalysts provided herein are superior to wide ranges of pre-existing Au-based, Pd-based, Pt-based, Cu-based, and Cu/Mn-based catalysts.

Embodiments of the catalysts provided herein were used successfully for CO oxidation under ambient reaction conditions as well as CO₂ reduction and could be extended for the conversion of other gases. In some aspects and embodiments, the catalysts provided herein possess various advantages over previously available catalysts for CO conversion reaction.

Porous polymeric 1-D gC₃N₄ materials were not typically used as catalysts for CO oxidation and/or CO₂ reduction, owing to inferior conductivity, inaccessible surface area, and weak interaction with the metal-catalysts. In some aspects and embodiments presented herein, atomic doping alters the physiochemical merits of porous 1-D gC₃N₄, including enhancement of its surface area to (320.6 m² g⁻¹), porosity (average pore size of 54.8 nm and pore volume 0.54 cc/g), and conductivity (doping with Pd/Cu, Au/Pd, and Pt/Pd).

In some embodiments, the method provided herein allowed one-pot synthesis of porous 1-D gC₃N₄ nanotubes, nanofibers, nanowires, and nanorods with high mass production, without the needing for template and multiple complicated steps. In addition, these nanostructures can be in suite doped with binary metals without any additional steps for doping or activation. In some embodiments, the method provided herein can be used easily for the fabrication of wide ranges of graphite-like carbon nitride materials in different shapes or dimensions doped with various metals.

In some embodiments, polymeric porous 1-D gC₃N₄ as provided herein can be simply prepared in a high yield from wide ranges of cheap and abundant resources as well as easily stored, handled, and modified without special laboratory equipment.

In some embodiments, atomic doping of Au/Pd, Pd/Cu, and Pt/Pd inside gC₃N₄ not only reduce the metal consumption, but also lessen the reaction barriers for CO oxidation or CO₂ reduction.

In some embodiments, the 1-D gC₃N₄ catalysts doped with only 1%-1.2 wt. % of Au/Pd, Pt/Pd, or Pd/Cu atoms provided herein were successfully used for complete CO conversion to CO₂ under temperature lower than 150° C.

In some embodiments, the 1-D gC₃N₄ catalysts provided herein can promptly oxidize CO thermally, electrochemically, photocatalytically, and photo-electrochemically to CO₂ along with reducing CO₂ to hydrocarbons (under low potential).

In some aspects and embodiments, provided herein are graphitic-like carbon nitride (gC₃N₄) nanostructures which are optionally doped atomically with one or more metal elements, and catalysts comprising the graphitic-like carbon nitride (gC₃N₄) nanostructures.

In some embodiments, the graphitic-like carbon nitride (gC₃N₄) nanostructure is doped atomically with one or more metal elements.

In some embodiments, the graphitic-like carbon nitride (gC₃N₄) nanostructure is not doped atomically with one or more metal elements.

In some embodiments, the carbon nitride nanostructure is nanotube, nanowire, nanorod, or nanofiber. In some embodiments, the nanostructure comprises at least one of nanotubes, nanowires, nanorods, or nanofibers.

In some embodiments, the carbon nitride nanostructure is one-dimensional (1-D). In some embodiments, the carbon nitride nanostructure is two-dimensional (2-D). In some embodiments, the carbon nitride nanostructure is three-dimensional (3-D). Preferably, the carbon nitride nanostructure is one-dimensional (1-D).

In some embodiments, the carbon nitride nanostructure is used as catalyst for carbon monoxide (CO) oxidation reaction. In some embodiments, the carbon nitride nanostructure is used as catalyst for carbon dioxide (CO₂) reduction reaction. In some embodiments, in CO oxidation reaction CO is oxidized to CO₂. In some embodiments, in CO₂ reduction reaction, CO₂ is reduced to HCO₂H.

In some embodiments, the carbon nitride nanostructure is functionalized with metal-based nanoparticle(s), single-atom, metal oxide nanoparticle(s), or hybrid nanoparticle(s), and is capable of being used as catalyst for CO oxidation reaction and/or CO₂ reduction reaction. In some embodiments, the metal is in the form of dopants, signal-atom, nanoparticle, ion, oxide, or any combination thereof. In some embodiments, the metal is mono, binary, ternary, or hybrid metal, and is in the form of dopants, single-atom, nanoparticles, ions, oxides, or any combination thereof.

In some embodiments, the carbon nitride nanostructure is doped atomically with one or more metal elements selected from the group consisting of gold (Au), palladium (Pd), copper (Cu), platinum (Pt), and any combination thereof. In some embodiments, the carbon nitride nanostructure is doped atomically with Au and Pd. In some embodiments, the carbon nitride nanostructure is doped atomically with Pd and Cu. In some embodiments, the carbon nitride nanostructure is doped atomically with Pd and Pt. In some embodiments, the nanostructure is doped atomically with Pd and at least a second metal element selected from the group consisting of Au, Cu, and Pt.

In some embodiments, the carbon nitride nanostructure is doped with 1 wt. %-1.2 wt. % metal elements. In some embodiments, the carbon nitride nanostructure is doped with 1 wt. %-1.2 wt. % Au/Pd. In some embodiments, the carbon nitride nanostructure is doped with 1 wt. %-1.2 wt. % Pd/Cu. In some embodiments, the carbon nitride nanostructure is doped with 1 wt. %-1.2 wt. % Pt/Pd.

In some embodiments, the carbon nitride nanostructure is carbon nitride nanotube doped with Au and Pd (Au/Pd/gC₃N₄NT). In some embodiments, the carbon nitride nanostructure is carbon nitride nanofiber doped with Au and Pd (Au/Pd/gC₃N₄NF). In some embodiments, the carbon nitride nanostructure is carbon nitride nanorod doped with Pt and Pd (Pt/Pd/CN nanorod). In some embodiments, the carbon nitride nanostructure is carbon nitride nanotube doped with Pd and Cu (Pd/Cu/gC₃N₄NF).

In some embodiments, the carbon nitride nanostructure is porous, having surface area ranging from 300 m² g⁻¹ to 350 m² g⁻¹. In some embodiments, the carbon nitride nanostructure is porous, having average pore size/diameter ranging from 45 nm to 65 nm, and pore volume ranging from 0.45 cc/g to 0.65 cc/g. In some embodiments, the surface area is about 300, 310, 320, 330, 340, or 350 m² g⁻¹. In some embodiments, the nanostructure is porous, having surface area ranging from 300 m² g⁻¹ to 350 m² g⁻¹, an average pore size/diameter ranging from 45 nm to 65 nm, and a pore volume ranging from 0.45 cc/g to 0.65 cc/g

In some embodiments, the carbon nitride nanostructure is nanotube, having surface area ranging from 300 m² g⁻¹ to 350 m² g⁻¹, for example, the surface area is about 300, 310. 320, 330, 340, or 350 m² g⁻¹.

In some embodiments, the carbon nitride nanostructure is nanotube, having surface area ranging from 200 m² g⁻¹ to 280 m² g⁻¹, for example, the surface area is about 200, 210, 220, 230, 240, 250, 260, 270, or 280 m² g⁻¹.

In some embodiments, the carbon nitride nanostructure is nanofiber, having surface area ranging from 50 m² g⁻¹ to 120 m² g⁻¹, for example, the surface area is about 50, 60, 70, 80, 90, 100, 110, or 120 m² g⁻¹. In some embodiments, the carbon nitride nanostructure is nanofiber, having surface area ranging from 80 m² g⁻¹ to 100 m² g⁻¹, for example, the surface area is about 80, 90, or 100 m² g⁻¹.

In some embodiments, the carbon nitride nanostructure is nanorod, having surface area ranging from 100 m² g⁻¹ to 200 m² g⁻¹, for example, the surface area is about 100, 110, 120, 130, 140, 150, 160, 170, 180, 190, or 200 m² g⁻¹.

In some embodiments, the carbon nitride nanostructure has conductivity.

In some embodiments, the carbon nitride nanostructure is doped atomically with one or more metal elements selected from the group consisting of gold (Au), palladium (Pd), copper (Cu), platinum (Pt), and any combination thereof; or the carbon nitride nanostructure is doped atomically with Au and Pd, or Pd and Cu, or Pd and Pt; and the carbon nitride nanostructure is porous, having surface area ranging from 300 m² g⁻¹ to 350 m² g⁻¹, average pore size/diameter ranging from 45 nm to 65 nm, and pore volume ranging from 0.45 cc/g to 0.65 cc/g.

In some embodiments, the carbon nitride nanostructure provided herein is used or is capable of being used as catalyst for oxidizing CO in a CO gas mixture, and/or reducing CO₂ in a CO₂ gas mixture, at a temperature ranging from the room temperature to 300° C., wherein the CO gas mixture contains 1%-100% of CO, the CO₂ gas mixture contains 1%-100% of CO₂.

In some embodiments, the carbon nitride nanostructure is used or is capable of being used as catalyst for CO and CO₂ conversion to a hydrocarbon. In some embodiments, the conversion is thermal, electrochemical, photocatalytic and photo-electrochemical conversion.

In some embodiments, the carbon nitride nanostructure is used or is capable of being used as catalyst for CO₂ conversion to CO and/or to a hydrocarbon.

In some embodiments, the CO and CO₂ conversion is enhanced using at least one other metal support selected from the group consisting of metal oxides, molecular sieves, carbon supports, ceramic-based materials, clay-based materials, and promoters,

wherein the metal oxides is at least one of TiO₂, CeO₂, Fe₂O₃, SiO₂, or Fe₃O₄,

wherein the molecular sieves is a zeolite,

wherein the carbon supports is at least one of diamond, graphene, cellulose, lignin, and carbon nanotube,

wherein the ceramic-based materials comprises bioglass and hydroxyapatite,

wherein the clay-based materials comprises bentonite and halloysite, and wherein the promotors comprises KOH, HCl, HNO₃, and CH₃COOH.

In some aspects and embodiments, provided herein is a method of making a carbon nitride (gC₃N₄) nanostructure, the method comprising:

a. providing a metal salt solution comprising one or more metal salts and a first solvent;

b. adding melamine to the metal salt solution followed by adding an acid solution, optionally while stirring at a first temperature, thereby forming a precipitate;

c. washing the precipitate with a second solvent;

d. drying the washed precipitate at a second temperature, thereby obtaining a powder.

In some embodiments, the method further comprises annealing the powder (e.g., by annealing after drying).

In some embodiments of the method, the metal salt is selected from the group consisting of HAuCl₄.3H₂O, HAuCl₄, K₂PdCl₄, Na₂PdCl₄, K₂PtCl₄, Na₂PtCl₄, CuCl₂.2H₂O, CuCl₂, and any combination thereof. In some embodiments, the metal salt is selected from the noble metals group, the transition metals, and any combination thereof.

In some embodiments of the method, the first solvent is selected from the group consisting of ethylene glycol and isopropanol. In some embodiments, the first solvent is selected from the group consisting of ethylene glycol, isopropanol, and water.

In some embodiments of the method, the second solvent is selected from the group consisting of ethanol and isopropanol.

In some embodiments of the method, the first and second solvents are the same.

In some embodiments of the method, the acid is selected from the group consisting of HNO₃ and HCl. In some embodiments, the HCl is combined with NaNO₃.

In some embodiments of the method, the first temperature is room temperature.

In some embodiments of the method, the second temperature is ranging from about 70° C. to 100° C.

In some aspects, the embodiments of the method provided herein are used for making the carbon nitride nanostructures provided herein. In some embodiments of the method, the nanostructure is prepared from a nitrogen-rich precursor comprising urea, thiourea, cyanuric acid-based, cyandiamide, pyridine, or guanidine hydrochloride.

In some aspects, provided herein are carbon nitride nanostructures made by the embodiments of the method provided herein.

In some aspects, provided herein are catalysts comprising the carbon nitride nanostructures of embodiments provided herein. In some embodiments, the catalysts provided herein further comprises other carbon-based materials or metal oxides for CO gas and/or CO₂ gas conversions.

In some aspects and embodiments, provided herein is a method of oxidizing CO, comprising

providing a CO gas mixture containing 1-100% CO (e.g., at least 1% CO);

providing a catalyst provided herein to the CO gas mixture (e.g., providing a catalyst to contact the CO gas mixture, wherein the catalyst comprises the graphitic-like carbon nitride nanostructure as otherwise disclosed herein); and

performing CO oxidation reaction.

In some aspects and embodiments, provided herein is a method of reducing CO₂, comprising

providing a CO₂ gas mixture containing 1-100% CO (e.g., at least 1% CO);

providing a catalyst provided herein to the CO₂ gas mixture (e.g., providing a catalyst to contact the CO₂ gas mixture, wherein the catalyst comprises the graphitic-like carbon nitride nanostructure as otherwise disclosed herein); and

performing CO₂ reduction reaction.

In some embodiments, CO is oxidized to CO₂ in the CO oxidation reaction. In some embodiments, CO₂ is reduced to HCO₂H in CO₂ reduction reaction.

In some embodiments, the CO oxidation or the CO₂ reduction is performed under a condition wherein a UV-light is on the catalyst.

In some aspects and embodiments, the method further comprises converting CO or CO₂ to a hydrocarbon.

In some aspects and embodiments, provided herein is a method of converting CO or CO₂ to a hydrocarbon, comprising

providing a CO or CO₂ gas mixture containing 1-100% CO or CO₂;

providing a catalyst provided herein to the gas mixture; and

performing conversion reaction.

In some embodiments, the conversion is for thermal, electrochemical, photocatalytic, or photo-electrochemical conversion.

EXAMPLES

Materials

Material Description Source Nitric acid 70% (HNO₃) Sigma-Aldrich Chemie GmbH (Munich, Germany) Hydrochloric acid, 37% (HCl) Sigma-Aldrich Chemie GmbH (Munich, Germany) Ethylene glycol, 99.8% (HOCH₂CH₂OH) Sigma-Aldrich Chemie GmbH (Munich, Germany) Sodium nitrate, 99.0% (NaNO3) Sigma-Aldrich Chemie GmbH (Munich, Germany) Ethanol, 99.9% (C₂H₅OH) Sigma-Aldrich Chemie GmbH (Munich, Germany) Sodium nitrate, 99% (NaNO₃) Sigma-Aldrich Chemie GmbH (Munich, Germany) Melamine, 99.0% (C₃H₆N₆) Sigma-Aldrich Chemie GmbH (Munich, Germany) Gold(III) chloride trihydrate, 99.9% Sigma-Aldrich Chemie (HAuCl₄•3H₂O) GmbH (Munich, Germany) Potassium tetrachloropladinate (II), 99.9% Sigma-Aldrich Chemie (K₂PdCl₄) GmbH (Munich, Germany) Copper (II) chloride dihydrate, 99.9% Sigma-Aldrich Chemie (CuCl₄•2H₂O) GmbH (Munich, Germany) Potassium tetrachloroplatinate (II), 99.99% Sigma-Aldrich Chemie (K₂PtCl₄) GmbH (Munich, Germany)

Materials and Products Characterization:

The morphology and composition of the as-synthesized materials were determined with a scanning electron microscope (SEM, Hitachi S-4800, Hitachi, Tokyo, Japan), a transmission electron microscope (TEM, TecnaiG220, FEI, Hillsboro, Oreg., USA) equipped with an energy-dispersive spectrometer (EDS), a high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), elemental mapping, and high-resolution TEM (HRTEM).

X-ray photoelectron spectroscopy (XPS) was carried out with a Kratos Axis (Ultra DLD XPS Kratos, Manchester, U.K.) equipped with a monochromatic Al Ka radiation source (1486.6 eV) under a UHV environment (ca. 5×10⁻⁹ Torr).

The X-ray diffraction pattern (XRD) was recorded on an X-ray diffractometer (X'Pert-Pro MPD, PANalytical Co., Almelo, The Netherlands) using a Cu Ka X-ray source (λ=1.540598 A).

The N₂-physisorption isotherms were measured on a Quantachrome Autosorb-1 analyzer (Quantachrome Instrument Corporation, Boynton Beach, Fla., USA). Or, the N₂-physisorption isotherms were measured on a Quantachrome Autosorb 3.01 instrument (Quanta chrome instrument corporation, Boynton Beach, Fla., USA) at 77 K. Before the measurements the samples were initially degassed, e.g., for 24-hours at 323 K under vacuum.

The Fourier transform infrared spectra were recorded on a Thermo Nicolet Nexus 670 FTIR spectrometer (Thermo Scientific, Madison, Wis., USA).

The Raman spectra were recorded on a PerkinElmer RamanStation 400 spectrometer with a 785 nm laser as an excitation source.

Example 1: Au/Pd/gC₃N₄NTs (gC₃N₄ Nanotubes Doped with Au and Pd)

Materials: Gold (III) chloride trihydrate (HAuCl₄.3H₂O, 99.99%), potassium tetrachloropalladate (II) (K₂PdCl₄, 99.99%), melamine (99%), ethylene glycol, and nitric acid (70%) were obtained from Sigma-Aldrich Chemie GmbH (Munich, Germany).

Synthesis of Porous Au/Pd/gC₃N₄NTs: 1 gram of melamine was slowly added to 30 mL of an ethylene glycol solution containing 1 mL of HAuCl₄.3H₂O (20 mM) and 1 mL of K₂PdCl₄ (20 mM), followed by the dropwise addition of 70 mL of HNO₃ (0.1 M) while stirring at room temperature for 30 minutes. The as-formed yellowish precipitate was washed with ethanol and dried at 80° C. for 12 hours before annealing at 450° C. (3°/minute) for 2 hours. After cooling to room temperature, the final product was saved for further characterization.

Material Characterization: The morphology and composition of the as-synthesized materials were determined using methods and instructions set forth in the general description above.

CO Oxidation Reaction: The CO conversion reaction on Au/Pd/gC₃N₄NTs was benchmarked relative to that on gC₃N₄NTs in a fixed bed quartz tubular reactor connected to an online gas analyzer (IR200, Yokogawa, Japan). In particular, 50 mg of each catalyst was initially treated at 250° C. under an O₂ flow of 50 mL min⁻¹ for 1 hour, followed by a flow of H₂ (30 mL·min⁻¹) for 1 hour. Then the catalysts were exposed to a reactant gas mixture consisting of 4% CO and 20% O₂ and balanced with Ar with a total flow of 50 mL min⁻¹ while heating. The percentage of CO conversion (% CO) was calculated using the following equation

% CO=[(CO_(in)−CO_(out))]/CO_(in)×100

where CO_(in) is the input amount and CO_(out) is the output amount of CO.

The cyclic voltammogram measurements were carried out on a Gantry electrochemical analyzer (reference 3000, Gamry Co., USA), using a three-electrode cell including a platinum wire, Ag/AgCl, and glassy carbon (GC, 5 mm) as the counter, reference, and working electrodes, respectively. The GC electrodes were covered with 10 μg of each catalyst, followed by the addition of 5 μL of Nafion (0.05%), and left to dry before the measurements.

Results and Discussion: Scheme 1 (FIG. 2) displays the fabrication process of the Au/Pd/gC₃N₄NTs consisting of the polymerization of melamine in an ethylene glycol solution in the presence of both Pd and Au precursors with the assistance of nitric acid followed by carbonization. In particular, nitric acid resulted in polymerized melamine with the formation of melon sheets as revealed by the formation of a yellowish precipitate. The as-formed precipitate was filtered and washed with ethanol and then dried at 80° C. to remove any impurities. Then, the obtained yellowish powder was carbonized by thermal annealing at an elevated temperature. This clearly displays the inherent capacity toward high mass production of the Au/Pd/gC₃N₄NTs.

The SEM images displayed the formation of monodisperse, uniform one-dimensional (1-D) nanotube morphology (FIGS. 3A and 3B). The TEM image of Au/Pd/gC₃N₄NTs also revealed the production of porous nanotube structure with an average length of 1.3 μm and a width of 95 nm (FIGS. 3C and 3D).

The high-magnification TEM image of a single nanotube showed its smooth surface and well-defined porous interior (FIG. 4A). The HRTEM image of a randomly selected area from the nanotube wall showed that the wall was composed of a polycrystalline graphitic layer of nanostructures (FIG. 4B). In the meantime, the HRTEM image of the core area displayed the polycrystalline phase of carbon nanostructures having various nanosheet curvatures (FIG. 4C).

The resolved Fourier filtered lattice fringes (FFT) in the core area showed their twisting with multiple crystalline defects and lattice distortion (FIG. 4C, inset). These defects could plausibly be attributed to the codoping effect with both Au and Pd. The interplanar distance among the adjacent lattice fringes was determined to be about 0.33 nm, which is assigned to the {002} facet of the graphitic carbon structure and has also been demonstrated by the corresponding Fourier transform (FT) pattern (FIG. 4C).

Both the TEM and SEM images showed only the presence of nanotube structure in the absence of any kind of undesired nanocrystals such as nanoparticles, which reflects on the uniformity of the as-formed Au/Pd/gC₃N₄NTs. Au and Pd could not be observed by TEM because neither of them are nanoparticles but are doped structures at the atomic level inside the carbon skeletal structure. Therefore, EDX, element mapping analysis, and XPS analysis were used to confirm the presence of Au and Pd as set forth below.

The HAADF-STEM image analysis also revealed the fruitful production of porous nanotube morphology with a smooth surface and well-defined thick walls (FIG. 5A). The average inner diameter of the nanotubes is about 70 nm and a wall thickness of 8 nm (FIG. 5A). The element mapping analysis was used to gain more insight into the composition of the materials thus obtained, which clearly depicted the obvious presence of C, N, Au, and Pd through the nanotubes (FIGS. 5B-5E). Both Au and Pd were found to be coherently distributed through the interior pores and exterior surface of the nanotube. The atomic contents of C, N, Au, and Pd were determined to be 40, 59, 0.52, and 0.48, respectively. These estimated atomic contents were almost in line with the initial precursor's concentrations, demonstrating the purity of the as-obtained materials.

EDX analysis was carried out to confirm the composition of the materials thus prepared. As expected, gC₃N₄NTs showed the presence of C and N with atomic ratios of 45 and 55, respectively, without any kind of metal impurities except for elemental Cu from the TEM copper grid (FIG. 5F). Meanwhile, Au/Pd/gC₃N₄NTs revealed the existence of C, N, Au, and Pd with atomic ratios of 41, 58, 0.51, and 0.49, respectively. This implies that the ratio of Au/Pd is almost 1, demonstrating their equal distribution in the as-made Au/Pd/gC₃N₄NTs. Meanwhile, the detection of Au and Pd by EDX analysis implies their presence in the bulk or inside the pore of the nanotubes. Therefore, both element mapping and EDX analysis of Au/Pd/gC₃N₄NTs displayed the homogeneous distribution of Au and Pd inside and outside the nanotubes.

Metal-free gC₃N₄NTs porous nanotubes were prepared as a control via activation of the polymerization and carbonization of melamine in an ethylene glycol solution free of Au and Pd precursors. Porous gC₃N₄NTs nanotubes with an average length of 1.21 μm and an average width of 93 nm were formed. The average width of the as-fabricated gC₃N₄NTs was slightly narrower than that of its counterpart Au/Pd/gC₃N₄NTs, which may be attributed to the Au and Pd dopants resulting in an expansion of the lattice.

The crystallinity of the as-synthesized Au/Pd/gC₃N₄NTs and gC₃N₄NTs was investigated by XRD analysis, which revealed that the diffraction peak at 20 of 27° corresponds to the {002} facet of graphitic-like carbon (FIG. 6A). The diffraction peak of Au/Pd/gC₃N₄NTs was positively shifted relative to gC₃N₄NTs, which can serve as indirect evidence of the Au and Pd dopants. The diffraction peaks for Au, Pd, and/or their oxides, were not resolved, likely owing to their low content and the inherent doping at the atomic level in Au/Pd/gC₃N₄NTs.

The electronic structure and surface composition of the Au/Pd/gC₃N₄NTs and gC₃N₄NTs were investigated by XPS analysis. The full-scan XPS indicated the presence of C, N, Au, and Pd peaks in Au/Pd/gC₃N₄NTs; meanwhile, gC₃N₄NTs showed only the C and N peaks (FIG. 6B). The binding energies of both C 1s and N 1s peaks in Au/Pd/gC₃N₄NTs were slightly positively shifted relative to their counterparts in gC₃N₄NTs, which could be due to the codoping effect with Au and Pd. The C 1s peak was fitted into two main peaks that were attributed to either graphitic carbon (C—C or C═C) at 284.7 eV or the sp² carbon-nitrogen (N—C═N) at 286 eV of aromatic rings such as the s-triazine unit and pyridine-like structure (FIG. 6C). The N 1s spectrum had been deconvoluted into three peaks that were attributed to the pyridinic N at 398.3 eV, pyrrolic N at 399.1 eV, and graphitic N at 401.1 eV, which are ubiquitous features of the carbon nitride-based materials (FIG. 6D). The high-resolution XPS of Pd 4f displayed two major peaks for Pd 3d_(5/2) at 335.0 V and Pd 3d_(3/2) at 340.5 eV with inferior oxide phases (FIG. 6F). Similarly, Au 4f showed two main peaks at 84.3 eV for Au 4f_(7/2) and 88.2 eV for Au 4f_(5/2) alongside insignificant oxide phases (FIG. 6E). The surface composition of the Au/Pd/gC₃N₄NTs showed the presence of C, N, Au, and Pd with atomic contents of 40, 59, 0.51, and 0.49, respectively, which confirms the presence of Au and Pd over the surface of the nanotubes. Both Au and Pd were also detected after etching of around 20 nm from the surface of the synthesized Au/Pd/gC₃N₄NTs, which indicates the presence of Au and Pd inside the nanotubes. This result was in line with the EDX and element mapping analysis, which revealed the homogeneous atomic distribution of Au and Pd inside and outside the nanotubes.

FIGS. 7A-7D shows the N₂-physisorption isotherm measurements of Au/Pd/gC₃N₄NTs compared to gC₃N₄NTs. The surface area and porosity of the as-fabricated materials were calculated using density functional theory (DFT). The surface area of Au/Pd/gC₃N₄NTs (320.6 m² g⁻¹) (FIG. 7A) was slightly larger than that of metal-free gC₃N₄NTs (275.7 m² g⁻¹) (FIG. 7C). This indicates that Au/Pd/gC₃N₄NTs could provide more active catalytic sites for the adsorption of reactant molecules during the catalytic reactions. The average pore diameter of Au/Pd/gC₃N₄NTs (57.6 nm) (FIG. 7B) was close to that of gC₃N₄NTs (54.8 nm) (FIG. 7D). This is in addition to other pores with different diameters in both samples. The porosity of Au/Pd/gC₃N₄NT nanotubes was expected to accelerate mass transfer and electron mobility during the catalytic applications.

The formation of Au/Pd/gC₃N₄NT and gC₃N₄NT had been additionally confirmed by FTIR analysis, where both samples depict the main absorption peaks assigned to the breathing mode of triazine at 810 cm¹ and the stretching mode of C—N heterocycles at 1200-1650 cm¹ (FIG. 8). In the meantime, the small broad bands between 2900 and 3300 cm⁻¹ could be possibly attributed to N—H vibrations that originated from the uncondensed amine groups. Noticeably, the bands at 2900-3300 cm⁻¹ were less intensive and broader in the Au/Pd/gC₃N₄NTs than those in its counterpart gC₃N₄NTs, most plausibly owing to the doping effect resulting from the strong attraction of N toward Au and Pd atoms (FIG. 8). This is indicated by the acceleration and improvement of the condemnation of the gC₃N₄NTs through Au/Pd-mediated synthesis. Additionally, the bands of Au/Pd/gC₃N₄NTs were slightly shifted relative to those of the gC₃N₄NTs, which is an apparent demonstration of the effect of codoping with Au and Pd.

The successful production of the as-synthesized materials was further proved by Raman spectroscopy using 785 nm laser light as an excitation source because it is one of the most accurate approaches to investigating the disorder in sp² carbon materials. Both gC₃N₄NTs and Au/Pd/gC₃N₄NTs displayed one prominent wide peak at 1550 cm⁻¹ attributed to the crystalline G band, in line with previous reports. Meanwhile, the absence of the disordered D-band indicates the high degree of graphitization of the as-synthesized materials that is ascribed to the amorphous/crystalline structure of the obtained materials without any resolved lattice fringes as investigated with HRTEM. Internally, both materials displayed a noticeable peak at 2680 cm⁻¹, which could be attributed to the symmetrical 2D, indicating their full dispersion.

Various reaction experiments were carried out to optimize the fabrication process of the Au/Pd/gC₃N₄NTs and to understand the reason behind the formation of nanotubes instead of other morphologies such as rods and sheets. The quick addition of melamine to the reaction solution drove the formation of aggregated flake-like nanostructures that implied the significant effect of the slow addition of melamine to produce uniform nanotube structure. The polymerization of melamine with nitric acid in the absence of ethylene glycol formed sheet-like aggregated structure. This is an unambiguous indication of the role of ethylene glycol as a structural directing agent in the formation of tubular structures. It was further proven through the use of isopropanol solution instead of ethylene glycol, which resulted in the formation of fiber-like architecture. The fast addition of nitric acid produced non-uniform and aggregated nanotubes. Uniform nanotubes atomically doped with Au and Pd were not obtained when the initial Au and Pd precursor ratios were changed.

The catalytic activity of the as-synthesized materials was investigated for the gas-phase CO oxidation reactions under ambient atmospheric conditions owing to the importance of this reaction in various fundamental, industrial, and environmental applications. The CO conversion to CO₂ (CO+½ O→CO₂) employing the Au/Pd/gC₃N₄NTs relative to that for metal-free gC₃N₄NTs was estimated at different reaction temperatures that ranged from room temperature to 400° C. (FIG. 9A). The results displayed that the metal-free gC₃N₄NTs did not exhibit any significant CO oxidation activity, even after heating, until 400° C.

After the codoping of gC₃N₄NTs with both Au and Pd, the CO conversion efficiency increased substantially with an increase in the reaction temperature until complete conversion occurred (FIG. 9A). The complete CO conversion (100%) was achieved on the Au/Pd/gC₃N₄NTs at 165° C., which indicated the significant doping effect in the improvement of the CO conversion efficiency when compared with gC₃N₄NTs. This is due to the electronic effect of Au and Pd which enhanced the adsorption of CO and activation of O₂ leading to oxygen accelerating the CO oxidation kinetics. This was indicated in the earlier conversion temperature of the Au/Pd/gC₃N₄NTs rather than in the gC₃N₄NTs (FIG. 9B). Indeed, the CO conversion on the Au/Pd/gC₃N₄NTs started at 74° C., which was significantly lower than that for the gC₃N₄NTs (130° C.). Following that, the CO conversion increased sharply on the Au/Pd/gC₃N₄NTs, whereas an increase in the reaction temperature did not afford any noticed increase in the CO conversion on the gC₃N₄NTs. This mainly could be attributed to the presence of oxygenated species formed by a combination between Au and Pd, which eventually leads to a decrease in the temperature required for the CO conversion. To this end, the half conversion temperature (T₅₀) and the full conversion temperature (T₁₀₀) on Au/Pd/gC₃N₄NTs were found to be 152° C. and 165° C., respectively. The quick CO conversion kinetics on the Au/Pd/gC₃N₄NTs gained further support by the observation of less time necessary for the full CO conversion. The CO conversion percentages on Au/gC₃N₄NTs and Pd/gC₃N₄NTs were about 8% and 7%, respectively, which were slightly higher than that of gC₃N₄NTs. This implies that gC₃N₄NTs should be codoped with both Au and Pd to achieve full CO conversion (100%), owing to the electronic and synergetic effects of Au/Pd.

The complete CO conversion time was noticed for the Au/Pd/gC₃N₄NTs at 30 minutes while at the same time the conversion was only 2% of CO for the gC₃N₄NTs (FIG. 9C). The T₁₀₀ of the newly synthesized Au/Pd/gC₃N₄NTs of 165° C. was lower than that previously reported for various Au-based and Pd-based catalysts such as Au_(0.75)Cu_(0.25)/SiO₂ of 300° C., Pd/La-doped γ-alumina of 175° C., Pt/CNx/SBA-15 of 250° C., and Pd-impeded 3-D porous graphene of 190° C. FIG. 9D illustrates the CO oxidation durability test carried out on the Au/Pd/gC₃N₄NTs for 10 durability cycles. The results showed that the Au/Pd/gC₃N₄NTs maintained their initial CO oxidation activity after 10 accelerated stability cycles without any significant loss. Additionally, after the durability test, Au/Pd/gC₃N₄NTs reserved their nanotube morphology without any noticeable cracking or agglomeration.

To elucidate the enhanced CO oxidation activity of the Au/Pd/gC₃N₄NTs, its ability to adsorb CO was measured. This includes benchmarking CO-stripping CVs in CO saturated in an aqueous solution of 0.1 M KOH at a scan rate of 50 mV s⁻¹ (FIG. 10A). The results displayed the superior CO-adsorption ability of the Au/Pd/gC₃N₄ NTs as compared to that of the metal-free gC₃N₄NTs. Additionally, the onset potential and oxidation potential of CO on Au/Pd/gC₃N₄NTs manifested a substantial negative shift relative to the gC₃N₄NTs. Meanwhile, the CO oxidation kinetics for the Au/Pd/gC₃N₄NTs was significantly faster than that for the gC₃N₄NTs under any applied potential, as indicated by the dashed lines in FIG. 10B.

These results unambiguously showed the superior CO oxidation activity and durability of the Au/Pd/gC₃N₄NTs, which was attributed to a unique combination of the intrinsic physicochemical properties of the gC₃N₄NTs and the impressive inherent catalytic merits of doping with Au and Pd. Indeed, the electronic effect of Au/Pd led to an enhancement of both the CO and O₂ adsorption along with the activation of O₂ that consequently led to an acceleration of the CO oxidation kinetics. Meanwhile, gC₃N₄NTs with its great electron density and oxidation activity. The porous nanotube architecture provides a highly accessible surface area and various active sites for the adsorption of the reactant molecules along with accelerating their transfer and molecular mobility.

Conclusion: In brief, this example sets forth a scaled-up approach to tailoring the fabrication of the Au/Pd/gC₃N₄NTs via the polymerization of melamine in an ethylene glycol solution that contains Pd and Au precursors with the addition of nitric acid followed by subsequent carbonization at high temperatures. In contrast to previous synthesis approaches to gC₃N₄, the present inventors' method is simple and allows for the high mass production of the gC₃N₄ porous nanotubes codoped with Au and Pd having a high surface area. These unique merits endow the Au/Pd/gC₃N₄NTs with respect to the CO oxidation activity with a significantly low T₁₀₀ of 165° C.

Example 2: Au/Pd/gC₃N₄NFs (gC₃N₄ Nanofibers Doped with Au and Pd)

Chemicals and Materials: HAuCl₄ (99.99%), K₂PdCl₄ (99.99%), melamine (99%), isopropanol (99.7%), and nitric acid (HNO₃ (70%)) were obtained from Sigma-Aldrich Chemie GmbH (Munich, Germany).

Synthesis of Au/Pd/gC₃N₄NFs: One-dimensional (1-D) Au/Pd/gC₃N₄NFs nanostructures were prepared by a sluggish mixing of 1 gram of melamine in 30 mL of isopropanol solution containing 1.5 mL of HAuCl₄ (20 mM) and 1.5 mL of K₂PdCl₄ (20 mM), followed by slow addition of 60 mL of HNO₃ (0.3 M) under stirring at 40° C. The obtained yellowish precipitate was washed with isopropanol and dried at 100° C. for 12 hours. The as-dried powder was then annealed at 480° C. for 2 hours at a heating rate of 5° C./min under N₂ in a chemical vapor deposition (CVD) furnace. The reference samples including Au/gC₃N₄, Pd/gC₃N₄, and gC₃N₄ were prepared by the same procedure, but with using reaction conditions with HAuCl₄, Na₂PdCl₄, and without metal precursors, respectively.

Materials Characterization: The morphology and composition of the as-obtained materials were determined using the methods and instructions set forth in the general description above. The N₂-physisorption isotherms were measured on a Quantachrome Autosorb 3.01 instrument (Quanta chrome instrument corporation, Boynton Beach, Fla., USA) at 77 K. Before the measurements, the samples were initially degassed for 24 hours at 323 K under vacuum.

CO Oxidation Reaction: A 50 mg of each catalyst was pretreated at 250° C. (5° C./min) under an O₂ flow of 50 mL min⁻¹ for 1 hour in a fixed bed quartz tubular reactor, followed by a flow of 5% H₂ in 95% He for 20 minutes. Then, the catalysts were exposed to the gas mixture consists of CO (4%), O₂ (20%), and Ar (76%) with a total flow of 50 mL min⁻¹. The CO conversion efficiency to CO₂ was estimated using an online gas analyzer (IR200, Yokogawa, Japan). The CO oxidation reaction was carried out at a temperature ranged from room temperature to 400° C. with a constant heating rate of 5° C./min using a programmed oven. The percentage of CO conversion (CO %) was calculated using the following equation:

CO %=[(CO_(in)−CO_(out))]/CO_(in)×100

where CO_(in) is the input quantity and CO_(out) is the output quantity.

Results and Discussion: Au/Pd/gC₃N₄NFs were prepared via polymerization of melamine in isopropanol solution contains Pd-precursor and Au-precursor by nitric acid followed by carbonization at 450° C. (Scheme 2, as shown in FIG. 11).

The SEM image of the Au/Pd/gC₃N₄NFs is shown in FIG. 12a . The obtained materials were formed in a high yield of uniform fiber-like nanostructure (FIG. 12A). The TEM image also showed the formation of one-dimensional nanofiber morphology without any undesired byproducts (FIG. 12B). The average dimensions of the obtained nanofiber were about 10±1 μm in length and 80±2 nm in width.

The high-magnification TEM of an individual nanofiber revealed its one-dimensional structure with a smooth surface (FIG. 13A). The high-resolution TEM (HRTEM) image of a selected area from the fiber wall revealed its graphitic layer structure (FIG. 13B). Meanwhile, the HRTEM of the core area depicted the random orientations of the lattice fringes with a polycrystalline and amorphous phase (FIG. 13C). Interestingly, the resolved graphite fringes in the core area contain various curvatures, which plausibly originated from their atomic doping with Au and Pd. Meanwhile, the estimated interplanar spacing in the fiber wall was nearly 0.336 nm, attributed to the {002} facet of graphitic-like carbon. Both SEM and TEM images indicated the uniformity of the as-formed Au/Pd/gC₃N₄NFs.

The HAADF-STEM of Au/Pd/gC₃N₄NFs also confirmed its one-dimensional nanofiber structure (FIG. 14A). The element mapping analysis of an individual nanofiber raveled only the consistent presence of C, N, Au, and Pd (FIGS. 14B-14E), indicated the uniformity of the as-formed Au/Pd/gC₃N₄NFs. The atomic ratio of C/N and Au/Pd are about 0.67 and 1.2, respectively. The EDX revealed the presence of C, N, Au, and Pd in the as-obtained Au/Pd/gC₃N₄NFs with atomic ratios of 40/59/0.55/0.45 (FIG. 14F).

As a reference, metal-free gC₃N₁NFs nanofibers were prepared by dissolving melamine in isopropanol followed by activation with nitric acid and annealing at 480° C. The as-formed gC₃N₄NFs have a one-dimensional nanofiber morphology with an average length of 10 μm and a width of 79±1 nm. The determined width of metal-free gC₃N₄NFs was slightly smaller than that of Au/Pd/gC₃N₄NFs, attributed to the presence of Au and Pd inside the skeleton structure of gC₃N₄NFs, which causes a slight lattice expansion.

FIG. 15A displays the XRD diffraction patterns of Au/Pd/gC₃N₄NFs and gC₃N₄NFs, which both revealed only one main diffraction peak attributed to {002} facet of the graphitic-like carbon structure. Interestingly, the diffraction peak of Au/Pd/gC₃N₄NFs was slightly shifted towards a higher 26 value relative to gC₃N₄NFs, due to the Au and Pd dopants. The absence of any diffraction peaks for Au and Pd are attributed to their atomic doping along with their low concentration.

The XPS survey of Au/Pd/gC₃N₄NFs displayed the presence of C 1s, N 1s, Au 4f, and Pd 3d peaks; meanwhile, gC₃N₄NFs only showed C 1s and N 1s peaks (FIG. 15B). The C 1s and N 1s peaks of Au/Pd/gC₃N₄NFs were slightly negatively shifted relative to their counterparts in gC₃N₄NFs, ascribed to the Au and Pd dopants. The deconvolution of the C 1s peak showed three peaks attributed to sp² graphitic carbon (C—C/C═C) at 284.6 eV, sp² carbon bonded nitrogen (C—N) at 286.3 eV, and sp²-carbon bonded nitrogen of the aromatic rings such as the s-triazine (N—C═N) at 288.1 eV, (FIG. 15C). The N is spectra were fitted into three peaks corresponded to pyridinic-N at 398.6 eV, pyrrolic-N at 401.1 eV, and graphitic-N at 401.7 eV (FIG. 15D), which are the main features for carbon nitrides materials. Au 4f revealed two peaks at 84.1 eV for Au 4f_(7/2) and 88.1 eV for Au 4f_(5/2) (FIG. 15E). Meanwhile, Pd 4f showed two peaks assigned to Pd 3d_(5/2) at 335.0 V and Pd 3d_(3/2) at 340.5 eV with the absence of any metal oxide phases (FIG. 15F). This indicates that Au and Pd dopants are in the metallic state phase. The surface atomic ratios of C/N/Au/Pd were estimated to be 41/58/0.5/0.5, respectively, impling that the atomic ratio of Au/Pd is 1/1 as in their precursors. In some aspects, the presence of Au and Pd dopants with the same atomic ratio (1/1) in the final product is the optimum Au/Pd ratio to balance strong catalytic activity and durability during the CO oxidation reaction. This is because Pd possesses a great affinity for CO adsorption and a high tolerance for the adsorption of CO₂ product, while Au provides a great adsorption/dissociation capability for 02. Thus, the ratio of Au/Pd should be the same to make them contribute equally during the CO oxidation reaction.

The FTIR analysis also confirmed the successful production of gC₃N₄NFs. The Raman analysis further proved the fabrication of gC₃N₄NFs doped with binary metals.

The surface areas of prepared materials were determined using a Brunauer-Emmett-Teller (BET). Both Au/Pd/gC₃N₄NFs and gC₃N₄NFs show the isotherm feature close to the type II curve with a H₃ hysteresis loop, indicating the presence of mesopores as well as macropores. These pores originated from the assembly of the nanofiber in the form of network-like structures as indicated by the SEM images. The hysteresis loop isotherm of Au/Pd/gC₃N₄NFs is slightly higher than that of gC₃N₄NFs, implying its higher surface area. Thus, the estimated BET surface area of Au/Pd/gC₃N₄NFs (85 m² g⁻¹) (FIG. 16A) is higher than that of gC₃N₄NFs (72 m² g⁻¹) (FIG. 16B). The observed high surface areas for both Au/Pd/gC₃N₄NFs and gC₃N₄NF are due to their uniform one-dimensional fiber-like nanostructure. The greater surface area of Au/Pd/gC₃N₄NFs makes its surfaces more accessible to the reactant molecules along with providing various accessible active sites during the catalytic reactions.

Several control experiments were carried out to optimize the preparation process of Au/Pd/gC₃N₄NFs. Initially, the quick addition of melamine to isopropanol solution contains metal precursors formed aggregated nanostructures (FIG. 17A). Agglomerated nanofibers were formed by decreasing the concentration of nitric acid to 0.05 M (FIG. 17B). Meanwhile, the fast addition of nitric acid produced irregular nanosheets (FIG. 17C).

Compared with previous reports, the inventive Au/Pd/gC₃N₄NFs combines the unique physicochemical properties of gC₃N₄NFs (e.g., great surface area, stability, and electrical conductivity) and outstanding catalytic merits of both Au and Pd dopants.

The CO oxidation reaction catalytic performance of Au/Pd/gC₃N₄NFs was investigated relative to Au/gC₃N₄NFs, Pd/gC₃N₄NFs, and metal-free gC₃N₄NFs under atmospheric conditions. The mesh sizes of Au/Pd/gC₃N₄NFs, Au/Pd/gC₃N₄NFs, Au/Pd/gC₃N₄NFs, and gC₃N₄NFs were about 0.13, 0.12, 0.15, and 0.17 mm, respectively. FIG. 18A shows the CO conversion efficiencies of the as-synthesized materials under various temperatures ranging from 25 to 400° C. In particular, metal-free gC₃N₄NFs displayed only 5% CO conversion even after heating to 400° C. (FIG. 18A). Meanwhile, the CO conversion efficiency of Au/Pd/gG₃N₄NFs, Au/gC₃N₄NFs, and Pd/gC₃N₄NFs increased steadily with increased reaction temperature (FIG. 18A), indicating the significant effect of Au and Pd on enhancing the CO oxidation efficiency of gC₃N₄NFs. Notably, the % CO conversion on Au/Pd/gC₃N₄NFs was significantly superior to Au/gC₃N₄NFs and Pd/gC₃N₄NFs. Additionally, the CO oxidation kinetics on Au/Pd/gC₃N₄NFs is quicker than that on Pd/gC₃N₄NFs and Au/gC₃N₄NFs at any reaction temperature as indicated by the dashed lines in FIG. 18B. The complete CO conversion temperature (T₁₀₀) of Au/Pd/gC₃N₄NFs (144° C.) is lower than that of Pd/gC₃N₄NFs (191° C.) and Au/gC₃N₄NFs (205° C.) (FIGS. 18A and 18C). This result indicates that using Au/Pd with the ratio of (1/1) led to decreasing the T₁₀₀ by 47° C. and 61° C. than that of the (0/1) and (1/0) ratios, respectively. The synergetic effect between Au and Pd tunes the adsorption of CO reactant and desorption of reaction product, which results in balancing between strong activity and durability.

The CO oxidation activity of our newly synthesized Au/Pd/gC₃N₄NFs is superior to various previously reported Au-based and Pd-based catalysts, such as AuPd/TiO₂ and AuPd/SiO₂ (Table 1, comparison of the catalytic activity of the inventive catalyst with previously reported catalysts):

TABLE 1 Comparison of Catalytic Activity Catalyst Activity, T₁₀₀ Reference Au/Pd/gC₃N₄NFs 144° C. Invention Pd-impeded nanohole 190° C. Ref structured 3D porous graphene Au/Pd/TiO₂ 190° C. Ref Co/CN/SBA-15 175° C. Ref Au/Pd/SiO₂ 182° C. Ref Pt/CNx/SBA-15 250° C. Ref Cu₂O/C₃N₄ 200° C. Ref Au/C₃N₄/SBA-15 270° C. Ref AuPd@A1₃O₃ 200° C. Ref

The CO oxidation stability tests of Au/Pd/gC₃N₄NFs, Pd/gC₃N₄NFs, and Au/gC₃N₄NFs were investigated for 48 hours at their T₁₀₀. Notably, Au/Pd/gC₃N₄NFs reserved their initial CO conversion efficiency without any significant loss after 48 hours. Meanwhile, Pd/gC₃N₄NFs and Au/gC₃N₄NFs lost around 10% and 1% of their initial activity, respectively (FIG. 18D). Their CO oxidation performance was measured after the durability test to further confirm the stability of the designed catalysts. The results reflected that Au/Pd/gC₃N₄NFs, Pd/gC₃N₄NFs, and Au/gC₃N₄NFs kept around 97.1, 91, and 87% of their initial CO oxidation activity. These results indicate the superior stability of Au/Pd/gC₃N₄NFs to Pd/gC₃N₄NFs and Au/gC₃N₄NFs originating from its stable active catalytic sites. The lower durability of Au/gG₃N₄NFs compared to Au/Pd/gC₃N₄NFs and Pd/gC₃N₄NFs is plausibly attributable to its poisoning by CO and CO₂ product during CO oxidation reaction, which blocks its active catalytic sites with time. This could not be occurred in case of Au/Pd/gC₃N₄NFs, ascribed to the combination between Au and Pd into the gC₃N₄NFs, which enriching the adsorption/activation of O₂ and CO along with weakening the adsorption of the CO₂ product. The SEM and EDX characterizations were carried out after the durability tests to confirm the structural and compositional stability of Au/Pd/gC₃N₄NFs. The results showed that Au/Pd/gC₃N₄NFs completely reserved their fiber-like shape without any agglomeration and degradation, indicating its morphological stability (FIGS. 19A and 19B). The EDX analysis of Au/Pd/gCN₄NFs showed its compositional stability without any significant changes after the accelerated durability cycles.

The CO oxidation mechanism on Au/Pd/gC₃N₄NFs can follow the Langmuir-Hinshelwood reaction. The quick adsorption of CO along with simultaneous activation of O₂ molecules accelerating and facilitating the CO oxidation at a lower reaction temperature.

These results demonstrated the significant CO oxidation activity and stability of Au/Pd/gC₃N₄NFs than that of Pd/gC₃N₄NFs, Au/gC₃N₄NFs, and gC₃N₄NFs. That originated from one-dimensional nanofiber morphology with a high surface area that provides various accessible active sites for CO-adsorption and 02-activation. Meanwhile, the combination between Au and Pd and gC₃N₄NFs tuned the adsorption of the reactants along with retarding the adsorption of CO₂ product. The interaction among Au/Pd and N-atoms of gC₃N₄NFs balance strong CO activity and durability.

Example 3: Pt/Pd/CNs Nanorods

Chemicals and Materials: Potassium tetrachloroplatinate(II) (K₂PtCl₄, 99.99%), potassium tetrachloropallidate(II) (K₂PdCl₄, 99.99%), melamine (99%), ethylene glycol (C₂H₆O₂, 99.8%), sodium nitrate (NaNO₃, 99.99%), hydrochloric acid (HCl, 37%), and nitric acid (HNO₃, 70%) were obtained from Sigma-Aldrich Chemie GmbH (Munich, Germany). All the chemicals were of analytical grade and used as received.

Synthesis of Pt/Pd/CNs Nanorods: In a representative synthesis, 1 gram melamine was dispersed in an aqueous ethylene glycol solution (15 mL) and added to 15 mL ethylene glycol solution containing 0.5 ml K₂PdCl₄ (20 mM) and 0.5 mL K₂PtCl₄ (20 mM), which was followed by the addition of an aqueous solution of HCl (0.1 M) and NaNO₃ (0.1 M) under ultrasonic treatment at 30° C. for 1 hour. The resulting yellowish precipitate was washed with ethanol and dried at 100° C. for 8 hours before being annealed at 550° C. (10° min⁻¹) for 2 hours under nitrogen (N₂) and then cooled to room temperature to obtain the final product.

Materials Characterization: The morphology and composition of the as-obtained materials were determined using methods and instructions set forth in the general description above. In addition, the inductively coupled plasma optical emission spectrometry (ICP-OES) analysis was carried out on a Thermo Scientific iCAP6300 (Thermo Fisher Scientific, USA).

CO Oxidation Reaction: The electrochemical measurements were performed on a Gamry electrochemical analyzer (reference 3000, Gamry Co., USA), using a three-electrode cell involving a Pt wire, Ag/AgCl, and glassy carbon (GC, 5 mm) as a counter, reference, and working electrode, respectively. The GC electrodes were covered with 10 μg cm⁻² of each catalyst, followed by the addition of 5 μL Nafion (0.05%) and left to completely dry in an oven at 80° C. before the measurements. The gas mixture was composed of 4% CO, 20% O₂, and 76% argon. The electrochemical active surface area (ECSA) of the as-formed materials was calculated using the following equation:

ECSA=Q/(m×420)

wherein Q represents the charge in H_(upd) adsorption/desorption area obtained after the double-layer correction of the cyclic voltamogram (CV) curves between −1 and −0.7 V, m is the catalyst loading on the electrode surfaces, and 420 μC cm⁻² represents the charge required for monolayer adsorption of hydrogen on Pt/Pd surface. For the photoelectrochemical CO oxidation activity, a fluorine-doped tin oxide sheet (0.5 cm) was used as the working electrode and was covered with 10 μg cm⁻² of Pt/Pd/CNs to which subsequently 5 μL Nafion (0.05%) was added. An ozone-free xenon lamp (100 mW cm⁻², Abet Technologies, USA) was used as the light source.

Results and Discussion: Pt/Pd/CNs nanorods were prepared via the cumbersome self-assembly of protonated melamine in an ethylene glycol solution containing Pt and Pd precursors, which subsequently sintered into CN nanorods through the rolling-up mechanism (Scheme 3, as shown in FIG. 20). In particular, melamine was initially dispersed slowly in an aqueous solution of ethylene glycol (solubility: nearly 22.67 g L⁻¹ at 20° C.) and then added sluggishly to an ethylene glycol solution containing Pt/Pd precursors. The slow dropwise addition of NaNO₃ and HCl solutions together promptly protonates the NH₂ bases of melamine. The existence of both H⁺ and NO₃ ⁻ changes the polarity of the ethylene glycol solution to facilitate the protonation of melamine as can be seen in the prompt formation of a yellowish precipitate. The protonation process stimulates the polymerization process of melamine to a polymeric network-like structure that simultaneously rolls up to form a rod-like carbon morphology upon pyrolysis at 550° C. Intriguingly, the structure evolution of Pt/Pd/CNs nanorods could be easily monitored through the naked eye, as reflected in the color change of the reaction solution from white to a yellowish color after the protonation of melamine, then to a brownish color after the polymerization process, and finally to deep brown after annealing.

The SEM image shows the high-yield (nearly 100%) formation of the elegant ID nanorod morphology (FIG. 21A). The as-synthesized nanorods are uniformly distributed with an average longitude of 94±2 nm and an average width of 11±1 nm, which results in an aspect ratio of 8.5 (FIG. 21B). The TEM image displays the produced nanorod structures with rough surfaces (FIG. 21C). Intriguingly enough, the nanorods have twined ends as indicated by the arrows in FIG. 21C. The absence of any undesired nanostructures, such as sheets and/or spherical nanostructures, indicates the purity of thus obtained materials. The selected area electron diffraction pattern (SAED) reveals diffraction rings corresponding to the {002} and {100} facets of the graphitic CN nanostructure inferring the polycrystalline structure of the CN nanorods (FIG. 21D)). The high-magnification TEM image of an individual nanorod clearly shows its 1D flat nanorod shape with a rough surface and a twined end (FIG. 22A). The HRTEM image of a randomly selected area from the shell area indicates that the shell is slightly twisted and composed of multiple graphitic layer structures with sub-amorphous layers, as indicated by the arrows in FIG. 22B. Meanwhile, the lattice fringes are continuous across the rod-shell, albeit the presence of some defects and lattice distortion as well, which might be ascribed to the codoping effect with Pt and Pd. The measured interplanar lattice spacing was nearly 0.336 nm, assigned to the {002} crystal plane of the graphitic CN structure. The Fourier filtered HRTEM (FFT) image of the marked area from the shell reveals its multiple layered structures, shown as the inset in FIG. 22B. The HRTEM image of a selected area from the core depicts the amorphous-crystalline structure of the graphitic CNs, although the presence of a distorted hexagonal carbon structure (FIG. 22C) is also observed. This was additionally proven by the FFT image, which displays the presence of hexagonal ring-like carbon of triazine along with various distorted rings (FIG. 22C). The HRTEM image also reflects the absence of any lattice fringes for Pt and Pd metals or their oxide phases. This indicates the atomic doping with Pt/Pd in the lattice structure of CNs, which accords with the previous reports on metal-doped supports such as Pd-doped gC₃N₄ nanosheets, Au/Pd-doped gC₃N₄, and Pt-doped TiO₂. The EDX analysis was carried out to investigate the composition of the materials thus obtained, which displayed the existence of C, N, Pt, and Pd with average contents of 38, 60.5, 0.8, and 0.7%, respectively, implying the formation of the C₃₈N_(60.5)Pt_(0.8)Pd_(0.7), nanostructure. The ratio of Pt/Pd determined by ICP-OES was nearly 1.14/1, which was in line with the EDX analysis. Notably, the absence of any undesired byproducts such as metal oxides in the as-synthesized nanorods indicates their uniformity.

The fruitful formation of 1-D CN nanorods was further evidenced in the HAADF-STEM image, which depicts that the nanorod has a rough surface with a twined end, in line with the SEM and TEM images (FIG. 23A). The element mapping analysis shows the coherent distribution of C, N, Pt, and Pd in the as-formed nanorods (FIGS. 23B-23E). The homogenous distribution of Pt and Pd implies their successful atomic doping inside the CN structure. The determined ratios of C/N/Pt/Pd were about 39/59.2/0.7/0.8, respectively, in line with that obtained by the EDX and ICP-OES analysis, which infers the uniformity of the nanorods thus formed. Hence, the detection of Pt and Pd in the EDX, element mapping, and ICP-OES analysis implies their atomic doping in CN nanorods rather than segregation and/or agglomeration.

As a reference, metal-free CN nanorods were fabricated by the same method of Pt/Pd/CNs by fixing all other reaction conditions and parameters but without using Pt and Pd precursors. The SEM and TEM images showed the formation of the nanorod morphology with an average longitude of 94±3 nm and an average width of 10±1 nm. This indicates the slight increase in the width of Pt/Pd/CNs nanorods relative to CNs that was attributed to the Pt and Pd dopants, which expand the lattice structure of CNs and coincides with the previous reports on metal-doped CNs. The measured ratio of C/N in metal-free CN nanorods was about 38/62, correspondingly.

FIG. 24A shows the XRD spectra of the Pt/Pd/CNs nanorods compared to that of metal-free CN nanorods, both of which revealed a weak diffraction peak observed at 13.10 indexed as the (100) facet attributed to the in-plane structural packing motif of s-triazine-based graphitic CNs. Meanwhile, both materials exhibited a strong diffraction peak centered at 20 of 27.4° corresponding to the amorphous (002) facet that was assigned to the interplanar stacking peak of the conjugated aromatic segment of the graphitic CN structure (FIG. 24A). The observed XRD diffraction spectra of Pt/Pd/CNs nanorods were slightly shifted towards a higher angle value with a slight decrease in the overall intensity compared to their metal-free CN counterparts. This was plausibly ascribed to the co-doping effect with both Pt and Pd, which enlarged the crystalline domain size of CNs as could be seen in the broadening of the (002) peak compared to that of CNs. Also, owing to the lower content of Pt/Pd (1.5 wt %) and their atomic doping inside the CN lattice structure, no peaks were observed for Pt and Pd or their oxides, which was in line with previous reports on metal-doped CNs.

XPS was used for investigating the electronic configuration and the surface composition of the materials thus obtained. The XPS survey of Pt/Pd/CNs nanorods indicated the presence of C 1s, N 1s, Pt 4f, and Pd 3d spectra; meanwhile, the CN nanorods revealed only the C 1s and N is spectra (FIG. 24B). The estimated ratio of C/N/Pt/Pd in Pt/Pd/CNs nanorods was about 38/60.5/0.6/0.9, while the ratio of C/N in CN nanorods was about 39/61, in line with the EDX and element mapping analysis. These results from the EDX and element mapping analysis demonstrate the doping of Pt and Pd inside and outside the CN nanorods.

The high N-content in the thus formed nanorods originated from the polymerization of melamine, which contains ˜65% N in its structure. The C 1s and N 1s of Pt/Pd/CNs were blue-shifted with a slight broadening and lower intensities compared to their counterparts in CN nanorods, due to the co-doping effect with Pt and Pd, which generates various defects inside the lattice structure of CNs. The C 1s spectra of Pt/Pd/CNs were fitted into a major graphitic carbon (C—C or C═C) at 284.8 eV, sp³-hybridized carbon-nitrogen (C—N) at 286.4, and (N—C═N) at 288.2 eV (FIG. 24C). Analogously, the N is spectra were deconvoluted into pyridinic-N, pyrrolic-N, and graphitic N at 398.3, 399.1, and 401.4 eV, respectively, which are typical characteristics of graphitic CN-based materials (FIG. 24D). The Pt 4f displayed only Pt 4f_(7/2) at 71.22 eV and Pt 4f_(5/2) at 74.4 eV (FIG. 24E), whereas Pd 3d showed Pd 3d_(5/2) at 335.2 eV and Pd 3d_(3/2) at 340.6 eV without any kind of Pt oxide and/or Pd oxides (FIG. 24F), indicating their presence in the metallic state.

The FTIR spectra were obtained to confirm the molecular structure of the as-prepared Pt/Pd/CNs and CNs, both of which reveal strong absorption spectra at 810-870 cm⁻¹ and 1255-1480 cm⁻¹ ascribed to the stretching vibration mode of triazine and C—N heterocycles of the aromatic rings, respectively. Meanwhile, the observed bands at 1570-1634 cm⁻¹ were attributed to the presence of C═N bonds, whereas the resolved small and broad bands at 2900-3100 cm-corresponded to the stretching of the C—H group. Interestingly, the noticed vibration band at 467 cm⁻¹ in Pt/Pd/CNs was plausibly ascribed to the N bonded to the Pt or Pd metal, due to their strong binding affinity. This indicates the formation of CNs codoped with both Pt and Pd, which matches with the EDX, element mapping, and XPS results. Raman spectroscopy analysis was conducted to get more insight into the formation of Pt/Pd/CNs nanorods and CN nanorods, both of which show a broad spectrum located at 1560 cm⁻¹ of the graphitic (G) band assigned to the crystalline graphite sp² carbon. This indicates the high degree of graphitization of the as-obtained materials as further seen in the absence of the D band at 1360 cm⁻¹. The spectrum observed at 2690 cm⁻¹ was attributed to the G′ peak, originated from the disordered surface and the low alignment of the graphitic CN nanostructure, and was in concurrence with the HRTEM results. The surface areas of the as-synthesized Pt/Pd/CNs and CN nanorods were estimated using the Brunauer-Emmett-Teller (BET) method (FIG. 25). The results show that the calculated BET surface area of Pt/Pd/CNs (155.2 m² g⁻¹) was higher than that of the CN nanorods (149.2 m²g⁻¹) (FIG. 25). A high surface area of Pt/Pd/CNs nanorods is important for providing more active sites during the CO oxidation reaction. The surface area of the inventive Pt/Pd/CNs nanorods (155.2 m²g⁻¹) was significantly higher than that of C₃N₄-MU (50 m²g⁻¹), pristine CNs (20.21 m²g⁻¹) as well as other CN-based materials with different shapes.

Various approaches are currently available for the controlled synthesis of CNs with different morphologies; however, most previous reports focused on zero-dimensional nanostructures without much emphasis on 1-D nanostructures. In certain aspects and embodiments, the instant invention provides a versatile approach for the rational synthesis of 1-D CN nanotubes and nanowires, which exhibited outstanding catalytic activity towards CO conversion to CO₂. In certain aspects, the instant invention is tailored to the synthesis of 1-D Pt/Pd-doped CN nanorods with a twined end, e.g., through the protonation of melamine using NaNO₃ and HCl in a glycol-mediated solution in the presence of Pt and Pd precursors, followed by annealing into CN nanorods.

The obtained capacitance current density of Pt/Pd/CNs nanorods was higher than that of metal-free CNs and Pt/C catalyst, owing to the co-doping effect with Pt/Pd along with their synergetic effect. Interestingly, there were no resolved peaks of any kind for Pt/Pd oxides such as PdO and/or PtO, implying the stability of Pt and Pd against oxidation.

The ECSA of Pt/Pd/CNs nanorods (75 m² g⁻¹) was higher than that of CNs (68 m²g⁻¹) and Pt/C (64 m²g⁻¹), resulting from the combination between the co-doping effect of Pt/Pd and the unique structure/composition features of CN nanorods. FIG. 26A displays the CVs measured in a CO-saturated aqueous solution of 0.1 M KOH at 50 mV s⁻¹ at room temperature. All catalysts show the main voltammogram characteristics of the CO-oxidation reaction involving a sharp oxidation peak in the forward direction and a small plateau in the backward direction between −0.25 and −0.01 V (FIG. 26A). These voltammogram features could not be resolved when the CVs were measured without cleaning and/or CO striping. Obviously, Pt/Pd/CNs nanorods exhibited a higher oxidation current density than the commercial Pt/C catalyst and CN nanorods. FIG. 26B reveals the linear sweep voltarnmetry (LSV) measured in CO-saturated 0.1 M KOH at 50 mV s⁻¹, which shows that PtPd/CNs nanorods consume a lower potential to oxidize CO and produce a greater current density at any potential point compared to Pt/C catalyst and CN nanorods, as indicated by the dashed lines in FIG. 26B.

This infers the quick CO oxidation kinetics on Pt/Pd/CNs nanorods compared with the commercial Pt/C catalyst and CN nanorods, which can be ascribed to the Pt/Pd electronic effect and the physicochemical properties of CN nanorods.

The CO oxidation current density of Pt/Pd/CNs (14.75 mA cm⁻²) was almost 2.01 and 23.41 times greater than that of Pt/C (7.32 mA cm⁻²) and CNs (0.63 mA cm⁻²), respectively (FIG. 26C). Meanwhile, the CO oxidation potential on Pt/Pd/CNs (−0.19 V) was inferior to that on Pt/C (−0.11 V) by 0.08 V and to that on CN nanorods (−0.03 V) by 0.16 V (FIG. 26C). Intriguingly, the obtained CO oxidation current density on our developed Pt/Pd/CNs (14.75 mA cm⁻²) was significantly higher than that on the previously reported porous PtPdRu nanodendrites (12 mA cm⁻²), Pt/Pd alloy nanoparticles (11.2 mA cm⁻²), and Pt nanowires (0.12 mA cm²).

The CO oxidation activity was benchmarked at different scan rates to gain further insight on the electrocatalytic CO oxidation kinetics (FIGS. 27A-27F). The electrocatalytic CO oxidation currents on Pt/Pd/CNs (FIG. 27A), Pt/C catalyst (FIG. 27C), and CNs (FIG. 27E) increased with increase in the scan rates from 25-250 mV s⁻¹, correspondingly. Meanwhile, the plot of the obtained CO oxidation current densities against the square root of the scan rates depicts a linear relationship [i.e., Pt/Pd/CNs (FIG. 27B), Pt/C catalyst (FIG. 27D), and CNs (FIG. 27F)]. This shows that CO oxidation was controlled by the diffusion rates on the as-synthesized catalysts. From the Randles-Sevcik equation, the slope of Pt/Pd/CNs (0.08 mV dec⁻¹) was found to be significantly larger than that of Pt/C (0.038 mV dec⁻¹) and CN catalyst (0.003 mV dec⁻). This implies quicker transportation kinetics on Pt/Pd/CNs than on Pt/C and CN nanorods (FIGS. 27B, 27D, and 27F). To compare the CO oxidation performances of the as-synthesized materials, their mass activities were calculated via normalizing the obtained current densities to the loading amount of the catalyst at −0.19 V. The mass activity of PtPd/CNs (1.47 mA g⁻¹) was about 23.3 and 2.03 times higher than that of CNs (0.063 mA g⁻¹) and Pt/C (0.73 mA g⁻¹), respectively, owing to the coupling of the unique physicochemical properties of CN nanorods with the outstanding catalytic properties of Pt/Pd, which results in their maximized utilization in the CO oxidation reaction.

The chronoamperometry (I-T) tests were conducted in a CO-saturated aqueous solution of 0.1 M KOH at 50 mV s⁻¹ to confirm the durability of the materials thus formed (FIG. 28A). Interestingly, Pt/Pd/CNs nanorods reveal a sluggish current attenuation with much higher retention of the current after 1000 s compared to Pt/C catalyst and CN nanorods, respectively, demonstrating the greater durability of PtPd/CNs (FIG. 28A). Following the I-T measurements, the CVs were measured again in deoxygenated 0.1 M KOH solution at 50 mV s⁻¹ to recalculate the ECSA. The ECSA of Pt/Pd/CNs nanorods, CN nanorods, and Pt/C catalysts deteriorated by around 10, 30, and 28%, respectively, implying the greater stability of Pt/Pd/CNs nanorods (FIG. 26D). Moreover, after the durability cycles Pt/Pd/CNs reserved around 93% of its initial CO oxidation current density, which showed that it was substantially more durable than Pt/C catalyst (74%) and CN nanorods (68%) (FIGS. 28B-28D).

The photoelectrochemical CO oxidation activity of Pt/Pd/CNs nanorods was tested under UV-visible light irradiation. Intriguingly, the obtained CO oxidation current on Pt/Pd/CNs under light illumination (21.9 mA cm⁻²) was increased 1.48 times than that in dark (14.7 mA cm⁻²) (FIG. 29). This was owing to the outstanding photocatalytic merits of CNs with a mid bandgap (˜2.7 eV). Additionally, the combination between Pt/Pd and CNs may maximize the visible-light absorption and delay the recombination rate of the generated charge carriers during the photoelectrochemical CO oxidation reaction. Interestingly, the 1-D nanorod shape of Pt/Pd/CNs was fully maintained without any significant changes after the durability cycles, while the commercial Pt/C catalyst suffered from an obvious detachment and aggregation, as shown in the TEM images measured after the durability tests.

These results demonstrate the CO oxidation activity and durability of Pt/Pd/CNs as compared to that of Pt/C and CNs. Without being bound by theory, the main reasons behind the superior catalytic performance of Pt/Pd/CNs nanorods include the advantageous catalytic properties of Pt/Pd, including the electronic effect, strong adsorption affinity for CO, high tolerance for reaction intermediates/products, and the generation of oxygenated species accelerating the reaction kinetics. This is coupled with the advantangeous physicochemical properties of CNs, such as electron-rich density, high surface area, abundant active sites, and great conductivity. These merits combine to provide both good CO oxidation activity and durability.

Conclusion: In certain aspects and embodiments as exemplified above, the inventive method provides a facile controlled fabrication of CN nanorods codoped with Pt and Pd (Pt/Pd/CNs) via protonation of melamine in an ethylene glycol solution containing Pt precursor and Pd precursor with the assistance of NaNO₃ and HCl solutions through a rolling-up mechanism. The designed Pt/Pd/CNs were formed in a high yield of uniform 1D CN nanorods (˜50 nm) codoped with Pt and Pd (1.5 wt %) without a template and/or multiple complicated steps. The electrocatalytic CO oxidation activity of PtPd/CNs was 2.01 and 23.41 times higher than that of the commercial Pt/C catalyst and metal-free CN nanorods, respectively. Meanwhile, Pt/Pd/CNs nanorods retained around 93% of their CO oxidation activity after the durability tests in addition to their morphological stability that was fully maintained without any changes. Furthermore, under UV-vis light irradiation, the CO oxidation activity of Pt/Pd/CNs nanorods was enhanced 1.4 times compared to that in the dark, owing to the unique photocatalytic properties of CNs. In certain aspects, the method could be scaled up for tailored fabrication of CN-based materials for catalytic CO oxidation.

Example 4: Pd/Cu/gC₃N₄NTs

Chemicals and Materials: K₂PdCl₄, CuCl₂.6H₂O (99%), melamine (99%), ethylene glycol, and nitric acid (HNO₃ (70%)) were purchased from Sigma-Aldrich Chemie GmbH (Munich, Germany).

Synthesis of Porous Pd/Cu/C₃N₄NTs: Pd/Cu/C₃N₄NTs were synthesized according to the method of Example 1, with slight modifications. The method included the sluggish mixing 1 gram of melamine in 30 mL of ethylene glycol solution containing 20 mM of K₂PdCl₄ and 20 mM of CuCl₂ for 20 min; 60 mL of HNO₃ (0.1 M) was then added under stirring for 30 min. The obtained yellowish precipitate was washed with ethanol and dried at 80° C. for 12 hours, followed by annealing of the dried product at 550° C. (5° C./min) for 2 hours under nitrogen. Metal-free gC₃N₄NTs were prepared by the same method but without the addition of Pd and/or Cu precursors.

Materials Characterization: The morphology and composition of the as-prepared materials were determined using methods and instructions set forth in the examples and general description above.

Results and Discussion: Pd/Cu/C₃N₄NTs were fabricated by the polymerization of melamine in an ethylene glycol solution containing Pd- and Cu-precursors by nitric acid followed by subsequent carbonization. FIG. 31A displays the SEM image of Pd/Cu/C₃N₄NTs that had been obtained for a high yield of the nanotube morphology. The TEM image also confirmed the formation of nanotube structure with a well-defined porous interior (FIG. 31B). The nanotubes were monodispersed with an average length of 1.5 m and a width of 82±4 nm. The histogram for the particle size destruction showed that the average width of nanotubes is about 80 nm. The TEM of a single nanotube Pd/Cu/C₃N₄ showed its smooth surface, hollow interior of about 60±2 nm and thick wall of 10±1 nm (FIG. 31C). The HRTEM of randomly selected areas from the outer wall of Pd/Cu/C₃N₄ nanotube revealed its graphitic-like layered structure with an inner space of 2 nm as indicated by the arrows in (FIG. 31D). The estimated spacing between the adjacent graphite fringes was about 0.337 nm, attributing to the {002} facet of carbon. Meanwhile, the lattice fringes resolved from the core area were amorphous with multiple curvatures and randomly oriented in different directions (FIG. 31E). This could be plausibly attributed to the merging of both Pd and Cu inside the lattice structure of gC₃N₄NTs. The selected-area electron diffraction (SAED) pattern revealed the presence of {002} and {100} facets of graphitic-like carbon structure (FIG. 31F). The metal-free C₃N₄NTs were formed in a porous nanotube structure with an average length of 1.4 μm and a width of 79±2 nm.

The HAADF-STEM image further reflected the formation of a porous nanotube morphology with a smooth surface and thick walls (FIG. 32A). The element mapping analysis of Pd/Cu/gC₃N₄NTs depicted the homogenous distribution of Pd, Cu, C, and N with atomic ratios of 0.6, 0.4, 58, and 41, respectively (FIGS. 32B-32E). These atomic ratios were in line with those determined by the EDX, which indicated the formation of Pd/Cu/gC₃N₄NTs and metal-free gC₃N₄NTs (FIG. 32F).

The XRD of Pd/Cu/gC₃N₄NTs and gC₃N₄NTs displayed two diffraction peaks at 270 and 13.2° assigned to the {002} and {100} facets of gC₃N₄, indicating the polymerization of melamine (FIG. 33A). The diffraction peaks of Pd/Cu/gC₃N₄NTs were slightly shifted to higher angle values relative to pure gC₃N₄NTs, which emanated from the Pd and Cu dopants. Meanwhile, the diffraction peaks for Pd or Cu and their oxides in the as-synthesized Pd/Cu/gC₃N₄NTs could not be resolved because of their low dopant content. Such behavior agrees with previous reports on metal-doped carbon nitrides.

The XPS full scan revealed the presence of C 1s, N 1s, Pd 3d, and Cu 2p in Pd/Cu/gC₃N₄NTs, whereas gC₃N₄NTs showed only C 1s and N 1s (FIG. 33B). The fitted XPS spectra of C 1s showed three peaks assigned to graphite C—C/C═C (284.5), sp³C—N (286.1 eV), and sp² N—C—N═N (288.5 eV) (FIG. 33C). The N is spectra displayed three nitrogen-species peaks attributed to C—N═C (397.6 eV), tertiary nitrogen N—(C)₃ (398.4 eV), and graphitic-N (400.7 eV) (FIG. 33D), which are usually observed in the gC₃N₄-based materials. The Pd 4f showed two peaks corresponding to Pd 3d_(5/2) (335.2 eV) and Pd 3d_(3/2) (340.7 eV) (FIG. 33E), whereas, Cu 2p revealed Cu 2p_(3/2) (932.3 eV) and Cu2p_(1/2) (951.2 eV) (FIG. 33F), implied the successful doping of gC₃N₄ with both Pd and Cu. The surface atomic ratios of C/N/Pd/Cu in Pd/Cu/gC₃N₄NTs were about 58/40.9/0.5/0.6, respectively. The determined atomic ratio of Pd/Cu was almost 1/1, which is in line with the EDX and element mapping analysis, showing the uniformity of the thus-formed materials.

FIG. 34 shows the nitrogen-adsorption/desorption isotherms of Pd/Cu/gC₃N₄NTs and gC₃N₄NTs, which both displayed the isotherms feature close to the type I curve. The major adsorption occurred for the hysteresis loop at 0.45<P/Po<0.9 that could serve as a clear sign of the large void volume of the nanotubes. The surface areas determined using a Brunauer-Emmett-Teller (BET) method are (240 m² g⁻¹) for Pd/Cu/gC₃N₄NTs (FIG. 34A) and (230 m² g⁻¹) for gC₃N₄NTs (FIG. 34B). Pd/Cu/gC₃N₄NTs with its outstanding surface area can facilitate the adsorption of reactant molecules and providing more active sites for the gas conversion reactions. The pore-size distribution calculated using the Barrett-Joyner-Halenda (BJH) method was about 30-90 nm for Pd/Cu/gC₃N₄NTs (FIG. 34C) and 40-75 nm for gC₃N₄NTs (FIG. 34D). The large pore sizes of Pd/Cu/gC₃N₄NTs can lead to the maximization of the utilization of both Pd and Cu in catalytic reactions.

FIG. 35A shows the CO conversion percentage of the as-prepared materials as a function of temperature. The results confirmed the superior catalytic performance of Pd/Cu/gC₃N₄NTs than their counterpart nanocatalysts. Notably, gC₃N₄NTs showed no significant CO conversion event at 400° C., whereas after the incorporation of Pd and Cu into the gC₃N₄NTs, the CO conversion percentage showed a steady increase with an increase in the reaction temperatures until complete conversion occurred. The complete CO conversion occurred at temperatures on Pd/Cu/gC₃N₄NTs relative to Pd/gC₃N₄NTs and Cu/gC₃N₄NTs, which shows the significant effect of doping with Pd/Cu on the enhancement of the CO oxidation activity (FIG. 35A). Meanwhile, the CO oxidation activity on Pd/Cu/gC₃N₄NTs was superior to gC₃N₄NTs at any reaction temperature as shown by the dashed lines, implying fast CO oxidation kinetics on Pd/Cu/gC₃N₄NTs (FIG. 34B).

Further evidence was provided by observing a lower complete conversion temperature (T₁₀₀) of Pd/Cu/gC₃N₄NTs (154° C.) when compared to 210° C. for Pd/gC₃N₄NTs and (250° C.) Cu/gC₃N₄NTs (FIGS. 35A and 35B). The catalytic activity of the newly designed Pd/Cu/gC₃N₄NTs was significantly greater than those for previously reported Pd-based and Cu-based catalysts, such as Cu₂O/C₃N₄ and Pd-impeded 3D porous graphene, as well as CuOx and CuMn₂O₄ catalysts. The durability tests were measured for 10 consecutive cycles at the T₁₀₀ for each catalyst. The results showed superior durability of the Pd/Cu/gC₃N₄NTs when compared to Pd/gC₃N₄NTs and Cu/gC₃N₄NTs. Interestingly, there was no noticeable deterioration in the CO oxidation activity on Pd/Cu/gC₃N₄NTs even after the accelerated 10 durability cycles; meanwhile, Pd/gC₃N₄NTs and Cu/gC₃N₄NTs lost around 5 and 10% of their activity, respectively. The TEM image of Pd/Cu/gC₃N₄NTs preserved its porous nanotube morphology with no significant changes observed after the stability tests serves as a clear which is a sign of the structural stability.

The XRD of Pd/Cu/gC₃N₄NTs after the stability tests revealed the main diffraction peaks at 270 and 13.2° that are assigned to the {002} and {100} facets of gC₃N₄, implying the structural stability in line with the TEM image. No diffraction peaks for Pd, Cu, and their oxides were detected, showing that their stability inside Pd/Cu/gC₃N₄NTs is primarily attributed to the atomic level nature of their doping. To obtain further insight into the stability of Pd/Cu/gC₃N₄NTs, an EDX analysis was conducted after the durability tests. The EDX result displayed the presence of Pd, Cu, C, and N with atomic ratios of 0.6, 0.4, 59, and 40, respectively. These atomic ratios are almost in line with those determined before the durability tests with no substantial changes; thus showing their compositional stability.

The electrochemical CO₂ reduction activity of Pd/Cu/gC₃N₄NTs had been tested relative to metal-free gC₃N₄NTs. The cyclic voltammogram (CVs) curve of Pd/Cu/gC₃N₄NTs revealed quasi-rectangular voltammogram features that are commonly seen in carbon-based materials that indicates a higher capacitance current. On the other hand, the gC₃N₄NTs displayed no obvious capacitance current (FIG. 36A). FIG. 36B shows the CVs measured in CO₂-saturated 0.5 M NaHCO₃ at 50 mV s⁻¹, which displayed the superior electrochemical CO₂ reduction activity of Pd/Cu/gC₃N₄NTs than those for the gC₃N₄NTs. The linear sweep voltammogram (LSVs) displayed the superior produced reduction currents for Pd/Cu/gC₃N₄NTs at any applied potential relative to those for gC₃N₄NTs that depicts its quick reduction kinetics (FIG. 36C). The CO₂ reduction current on Pd/Cu/gC₃N₄NTs (8.31 μA cm⁻²) is 4 times higher than that for gC₃N₄NTs (2.1 μA cm⁻²). This originated from the higher conductivity and quick charge transfer on Pd/Cu/gC₃N₄NTs than with gC₃N₄NTs. Further proof was provided by the electrochemical impedance spectroscopy (EIS) (FIG. 36)).

UV-light irradiation enhanced the CO₂ reduction activity of Pd/Cu/gC₃N₄NTs substantially relative to gC₃N₄NTs (FIG. 36E). The photo-electrochemical current obtained for Pd/Cu/gC₃N₄NTs was 24.5 μA cm⁻² under light is 2.92 fold higher than that in the dark of 8.31 μA cm⁻². This is owing to the combination of semiconducting properties of gC₃N₄NTs with the catalytic merits of Pd/Cu. The CVs on Pd/Cu/gC₃N₄NTs were measured with different scan rates, which showed that the current density increased continuously with increasing the scan rates. Meanwhile, the Randles-Sevcik equation, showed a linear plot on Pd/Cu/gC₃N₄NTs that infers to the quick transportation kinetics. The durability tests showed that the Pd/Cu/gC₃N₄NTs preserved its CO₂ electrochemical reduction performance with no significant changes. The photo-electrochemical CO₂ reduction durability test showed that Pd/Cu/gC₃N₄NTs preserved its catalytic activity with no significant loss. The quantitative analysis for the CO₂ reduction products using gas chromatography analyzer displayed that, Pd/Cu/gC₃N₄NTs successfully reduced CO₂ to mainly formic acid (˜90%) and methanol (10%).

These results demonstrated the superior CO oxidation and CO₂ reduction performances of Pd/Cu/gC₃N₄NTs. This is plausibly attributed to the combination between the unique physicochemical properties of gC₃N₄NTs (i.e., high surface area, active sites, low density, abundant electron density, and chemical/thermal stability) as well as the superior catalytic features of Pd/Cu, such as high 02/CO adsorption affinity and vigorous adsorption/dissociation activities of O₂. Moreover, porous nanotube morphology not only speeds up the molecular mobilities, but also maximizes the utilization of both Pd and Cu during the catalytic reactions. Meanwhile, the presence of Pd and Cu along with abundant N-atoms in the skeleton structure of Pd/Cu/gC₃N₄NTs tunes the intense binding energies of the reactants along with exhibiting a demonstrated high tolerance for the reaction products.

Conclusions: In brief, provided herein is a versatile approach for the rational fabrication of porous gC₃N₄ nanotubes doped at the atomic level with Pd and Cu (Pd/Cu/gC₃N₄NTs) upon polymerization of melamine in ethylene glycol solution that contains the metal precursors in the presence of nitric acid followed by pyrolysis under nitrogen. The as-formed Pd/Cu/gC₃N₄NTs have well-defined porous nanotube structure with high surface area and doped with Pd and Cu. The gas phase CO oxidation activity and durability of Pd/Cu/gC₃N₄NTs were superior to Pd/gC₃N₄NTs, Cu/gC₃N₄NTs, and gC₃N₄NTs, respectively. The electro-chemical CO₂ reduction activity of Pd/Cu/gC₃N₄NTs was 4 times higher than that of gC₃N₄NTs. Meanwhile, the UV-light irradiation enhanced the CO₂ reduction performance of Pd/Cu/gC₃N₄NTs by 2.92-fold as compared to the case of under dark. In certain aspects, the instant application provides new routes for the rational design of C₃N₄NTs doped with multiple metal-based catalysts for multifarious applications.

The citations herein are incorporated in their entirety for all purposes, except insofar as their incorporation would create an inconsistency, the text of the present application shall control. 

What is claimed is:
 1. A graphitic-like carbon nitride nanostructure, wherein the nanostructure is doped atomically with one or more metal elements.
 2. The nanostructure of claim 1, wherein the nanostructure comprises at least one of nanotubes, nanowires, nanorods, or nanofibers.
 3. The nanostructure of claim 1, wherein the nanostructure is one-dimensional (1-D).
 4. The nanostructure of claim 1, wherein the nanostructure is functionalized with one or more metal-based nanoparticles, single atoms, metal oxide nanoparticles, or hybrid nanoparticles; and wherein the nanostructure is a catalyst for a CO oxidation reaction and/or a CO₂ reduction reaction.
 5. The graphitic-like carbon nitride nanostructure of claim 4, wherein the one or more metal elements are in the form of dopants, single atoms, nanoparticles, ions, oxide, or any combination thereof.
 6. The graphitic-like carbon nitride nanostructure of claim 1, wherein the nanostructure is doped atomically with at least one metal selected from the group consisting of gold (Au), palladium (Pd), copper (Cu), and platinum (Pt).
 7. The graphitic-like carbon nitride nanostructure of claim 6, wherein the nanostructure is doped atomically with Pd and at least a second metal element selected from the group consisting of Au, Cu, and Pt.
 8. The graphitic-like carbon nitride nanostructure of claim 7, wherein the nanostructure is doped with 1-1.2 wt. % Au/Pd, or 1-1.2 wt. % Pd/Cu, or 1-1.2 wt. % Pt/Pd.
 9. The graphitic-like carbon nitride nanostructure of claim 1, wherein the nanostructure is doped with 1-1.2 wt. % metal elements.
 10. The graphitic-like carbon nitride nanostructure of claim 1, wherein the nanostructure is selected from the group consisting of a carbon nitride nanotube doped with Au and Pd (Au/Pd/g C₃N₄NT); a carbon nitride nanofiber doped with Au and Pd (Au/Pd/gC₃N₄NF); a carbon nitride nanorod doped with Pt and Pd (Pt/Pd/CN nanorod); and a carbon nitride nanotube doped with Pd and Cu (Pd/Cu/gC₃N₄NF).
 11. The graphitic-like carbon nitride nanostructure of claim 1, wherein the nanostructure is porous, having surface area ranging from 300 m² g⁻¹ to 350 m² g⁻¹, an average pore size/diameter ranging from 45 nm to 65 nm, and a pore volume ranging from 0.45 cc/g to 0.65 cc/g.
 12. The graphitic-like carbon nitride nanostructure of claim 1, wherein the nanostructure is a catalyst for oxidizing CO in a CO gas mixture and/or reducing CO₂ in a CO₂ gas mixture at a temperature ranging from room temperature to 300° C., wherein the CO gas mixture contains 1%-100% of CO, and wherein the CO₂ gas mixture contains 1%-100% of CO₂.
 13. The graphitic-like carbon nitride nanostructure of claim 12, wherein the CO and CO₂ conversion is enhanced using at least one other metal support selected from the group consisting of metal oxides, molecular sieves, carbon supports, ceramic-based materials, clay-based materials, and promoters, wherein the metal oxides is at least one of TiO₂, CeO₂, Fe₂O₃, SiO₂, or Fe₃O₄, wherein the molecular sieves is a zeolite, wherein the carbon supports is at least one of diamond, graphene, cellulose, lignin, and carbon nanotube, wherein the ceramic-based materials comprises bioglass and hydroxyapatite, wherein the clay-based materials comprises bentonite and halloysite, and wherein the promotors comprises KOH, HCl, HNO₃, and CH₃COOH.
 14. A method of making a metal-doped carbon nitride (gC₃N₄) nanostructure, comprising: a. providing a metal salt solution comprising one or more metal salts and a first solvent; b. adding melamine to the metal salt solution followed by adding an acid solution, optionally while stirring at a first temperature, thereby forming a precipitate; c. washing the precipitate with a second solvent; d. drying the washed precipitate at a second temperature, thereby obtaining a powder.
 15. The method of claim 14, further comprising annealing the powder.
 16. The method of claim 14, wherein the metal salt is selected from the noble metals group, the transition metals, and any combination thereof.
 17. The method of claim 14, wherein the metal salt is selected from the group consisting of HAuCl₄.3H₂O, HAuCl₄, K₂PdCl₄, Na₂PdCl₄, K₂PtCl₄, Na₂PtCl₄, CuCl₂.2H₂O, CuCl₂, and any combination thereof.
 18. The method of claim 14, wherein the first solvent is selected from the group consisting of ethylene glycol, isopropanol, and water.
 19. The method of claim 14, wherein the second solvent is selected from the group consisting of ethanol and isopropanol.
 20. The method of claim 14, wherein the acid is selected from the group consisting of HNO₃, HCl, and a combination thereof.
 21. The method of claim 14, wherein the first temperature is room temperature.
 22. The method of claim 14, wherein the second temperature is ranging from about 70° C. to 100° C.
 23. The method of claim 14, wherein the nanostructure is prepared from a nitrogen-rich precursor comprising urea, thiourea, cyanuric acid-based, cyandiamide, pyridine, or guanidine hydrochloride.
 24. A method of oxidizing CO, comprising providing a CO gas mixture comprising 1-100% CO; providing a catalyst to contact the CO gas mixture, wherein the catalyst comprises the graphitic-like carbon nitride nanostructure of claim 1; and performing a CO oxidation reaction.
 25. The method of claim 24, wherein the CO is oxidized to CO₂.
 26. A method of reducing CO₂, comprising providing a CO₂ gas mixture comprising at least 1% CO; providing a catalyst to contact the CO₂ gas mixture, wherein the catalyst comprises the graphitic-like carbon nitride nanostructure of claim 1; and performing a CO₂ reduction reaction.
 27. The method of claim 26, wherein CO₂ is reduced to HCO₂H and CO is oxidized to CO₂.
 28. The method of claim 26, wherein the CO oxidation or the CO₂ reduction is performed under a condition wherein a UV-light is on the catalyst.
 29. The method of claim 26, comprising converting CO or CO₂ to a hydrocarbon. 